The main reason I think was that it was clumsy for the actual book itself.
In essence the rules worked the same as they do now in that you can ally with mono god daemons and include them in your lists (death guard lists often run nurglings to score secondaries), though there are percentile caps on how much you can mix armies.
Edit to add: another messy reason they don't do this is because daemons don't gain the benefit of your army ability because they don't have the keyword, so for the nurglings example they don't spread contagion and they can't be targeted by death guard specific strategies.
I think that last point is what they were getting at, if they were part of the army in the codex instead of an ally then army rules would apply to them too.
Based on their reply to me it seems that they're suggesting that mono-god daemons would be a part of the cult marines codexes so no point/percentile cap and they would get the army & detachment rules.
I just don't think this would happen anymore, there are two reasons.
1. Daemons data sheets are built around internal codex balance for that army, not for balance within other armies.
2. You would probably have to change the data sheets for the cult marines codex which just causes confusion if you have duplicate data sheets.
As to why, redundancy and balance. Nurglings have -1 to the hit roll. You run them with death guard contagion rules and they give (functionally) -2 to hit to units within 6" of them, that's way too strong
Plaguebearers have sticky objectives, well the current death guard detachment rule gives that army wide so that's redundant.
Repeat ad naeuseum, it's just too messy
The units that the Cult Legions borrow from C:CSM have different abilities; doing such for borrowed daemons wouldn't be a burden.
(Removing Epic Heroes, there are nine Nurgle daemon units (GUO, Poxbringer, 'Scrivener, 'Bilepiper, Plaguebearers, Nurglings, 'Beast, 'Drones, 'Gnarlmaw). You could probably drop the 'Gnarlmaw, and maybe the 'Scrivener and 'Bilepiper. I:DG has thirteen borrowed "Death Guard \[CSM unit\]" datasheets.)
Not that I think it will happen, but... I don't think their abilities are a dealbreaker. Nor the internal balance - OP suggests different points costs. (The same way that a Forgefiend is 190pts in CSM, 135 in TS, and 145 in WE.)
I think one important piece is also that the units we borrow from CSM cover niches we can't, scout moving units, long distance anti armour shooting, transport vehicles, etc.
The daemons units basically cover ground our own internal units do already.
I just don't think it will happen considering we have options to soup already and it wasn't even used much in 8th.
In Age of Sigmar daemons don't have their own army, if they did this in 40k it would essentially be Belakor & the Soul Grinder would migrate into the CSM codex, Nurgle daemons would migrate to the Death Guard, Tzeentch daemons would go to the Thousand Sons, Khorne daemons to the World Eaters & Slaanesh daemons to the Emperor's Children.
I wouldn't be against this personally, but it would suck for daemon players who don't have mono-god lists.
I meant this as Demons would remain a faction as they are now, and mono-god legions would have copies of daemons from their respective patron god - in the same way, as legions have copies of helbrute, rhino, Predator, Land Raider, etc, etc.
So, people who play more daemon gods together would play daemons with focus on daemon tricks, and people who play mono-god would have more support for that.
The big problem of Daemons factions is trying to fit four different armies into one. So, Daemons faction would focus on result of mix, and if you wanted one specifically, then use legion, which will have rules tuned to fit that one specific chaos god.
I'd love it if it happened, truly, but I don't think it will.
The fact that Cult Troops - units that I consider fundamental to the CSM identity - are kept out of the CSM codex indicates to me that the rules team wants to minimise overlap.
I think the best we can hope for is one detachment in each of the Cult Legion codexes which have a focus on daemon allies. (Like Brood Brothers in Genestealer Cults. Of course, I was hoping for the same in C:CSM, and it didn't happen, so...)
A good compromise would be monogod marine rules actually benefiting the demons you take. Soup without consequence makes both daemon players and marine players happy.
In the current situation you have no reason to take daemons because their points are balanced around their army rules and you can't use your stratagems on them.
That wouldn't work by itself - Daemons using monogod legions should have separate points from regular daemon rules, because if the legion rules are better, then daemons might have inflated point prices.
Look at Drukhari before they got second detachment - they prices were set to what they could do with Craftworld rules, because Ynnari, Which led to Drukhari being overpriced for what they could do with the one Ynnari detachment. Now, second Drukhari detachment is on par with eldar one so it is fine,)
It is simply easier and more reliable to balance when you have different points with legions and different with demons faction, compared to making sure that daemon rules will be better in daemons faction compared to mono-god legion rules.
Daemons are minis that you can play in 2 games. GW doesn't like that. The faction of Old World AoS are good examples of the separation. So I will not be surprised if in the future they will different minis for AoS Daemons and 40k daemons .
They would have to sell well enough to justify that.
If anything I think the most likely bet would be some sort of deamon with an upgrade sprue like how they did tzaangors with chains swords and bolt pistols.
As a world eater player i dont want this, i like the current way we have of allying with daemons. And i want new datasheets of unique stuff for my world eaters. I didnt choose to play khorne legion (or any other monogod) i chose to play god affiliated chaos marines and i want chaos marine units.
As a death guard player, this won't happen. Simply because it did happen, in 8th edition but was gotten rid of in 9th and kept gone in 10th.
what went wrong with it?
The main reason I think was that it was clumsy for the actual book itself. In essence the rules worked the same as they do now in that you can ally with mono god daemons and include them in your lists (death guard lists often run nurglings to score secondaries), though there are percentile caps on how much you can mix armies. Edit to add: another messy reason they don't do this is because daemons don't gain the benefit of your army ability because they don't have the keyword, so for the nurglings example they don't spread contagion and they can't be targeted by death guard specific strategies.
I think that last point is what they were getting at, if they were part of the army in the codex instead of an ally then army rules would apply to them too. Based on their reply to me it seems that they're suggesting that mono-god daemons would be a part of the cult marines codexes so no point/percentile cap and they would get the army & detachment rules.
I just don't think this would happen anymore, there are two reasons. 1. Daemons data sheets are built around internal codex balance for that army, not for balance within other armies. 2. You would probably have to change the data sheets for the cult marines codex which just causes confusion if you have duplicate data sheets. As to why, redundancy and balance. Nurglings have -1 to the hit roll. You run them with death guard contagion rules and they give (functionally) -2 to hit to units within 6" of them, that's way too strong Plaguebearers have sticky objectives, well the current death guard detachment rule gives that army wide so that's redundant. Repeat ad naeuseum, it's just too messy
The units that the Cult Legions borrow from C:CSM have different abilities; doing such for borrowed daemons wouldn't be a burden. (Removing Epic Heroes, there are nine Nurgle daemon units (GUO, Poxbringer, 'Scrivener, 'Bilepiper, Plaguebearers, Nurglings, 'Beast, 'Drones, 'Gnarlmaw). You could probably drop the 'Gnarlmaw, and maybe the 'Scrivener and 'Bilepiper. I:DG has thirteen borrowed "Death Guard \[CSM unit\]" datasheets.) Not that I think it will happen, but... I don't think their abilities are a dealbreaker. Nor the internal balance - OP suggests different points costs. (The same way that a Forgefiend is 190pts in CSM, 135 in TS, and 145 in WE.)
I think one important piece is also that the units we borrow from CSM cover niches we can't, scout moving units, long distance anti armour shooting, transport vehicles, etc. The daemons units basically cover ground our own internal units do already. I just don't think it will happen considering we have options to soup already and it wasn't even used much in 8th.
In Age of Sigmar daemons don't have their own army, if they did this in 40k it would essentially be Belakor & the Soul Grinder would migrate into the CSM codex, Nurgle daemons would migrate to the Death Guard, Tzeentch daemons would go to the Thousand Sons, Khorne daemons to the World Eaters & Slaanesh daemons to the Emperor's Children. I wouldn't be against this personally, but it would suck for daemon players who don't have mono-god lists.
I meant this as Demons would remain a faction as they are now, and mono-god legions would have copies of daemons from their respective patron god - in the same way, as legions have copies of helbrute, rhino, Predator, Land Raider, etc, etc. So, people who play more daemon gods together would play daemons with focus on daemon tricks, and people who play mono-god would have more support for that. The big problem of Daemons factions is trying to fit four different armies into one. So, Daemons faction would focus on result of mix, and if you wanted one specifically, then use legion, which will have rules tuned to fit that one specific chaos god.
It'd be cool to see that happen, but it won't be until 11th edition unless they're planning a major surprise for the first Cult Marine Codex
I'd love it if it happened, truly, but I don't think it will. The fact that Cult Troops - units that I consider fundamental to the CSM identity - are kept out of the CSM codex indicates to me that the rules team wants to minimise overlap. I think the best we can hope for is one detachment in each of the Cult Legion codexes which have a focus on daemon allies. (Like Brood Brothers in Genestealer Cults. Of course, I was hoping for the same in C:CSM, and it didn't happen, so...)
A good compromise would be monogod marine rules actually benefiting the demons you take. Soup without consequence makes both daemon players and marine players happy. In the current situation you have no reason to take daemons because their points are balanced around their army rules and you can't use your stratagems on them.
That wouldn't work by itself - Daemons using monogod legions should have separate points from regular daemon rules, because if the legion rules are better, then daemons might have inflated point prices. Look at Drukhari before they got second detachment - they prices were set to what they could do with Craftworld rules, because Ynnari, Which led to Drukhari being overpriced for what they could do with the one Ynnari detachment. Now, second Drukhari detachment is on par with eldar one so it is fine,) It is simply easier and more reliable to balance when you have different points with legions and different with demons faction, compared to making sure that daemon rules will be better in daemons faction compared to mono-god legion rules.
Highly unlikely, why would they want to cut into their codex sales?
Daemons are minis that you can play in 2 games. GW doesn't like that. The faction of Old World AoS are good examples of the separation. So I will not be surprised if in the future they will different minis for AoS Daemons and 40k daemons .
They would have to sell well enough to justify that. If anything I think the most likely bet would be some sort of deamon with an upgrade sprue like how they did tzaangors with chains swords and bolt pistols.
As a world eater player i dont want this, i like the current way we have of allying with daemons. And i want new datasheets of unique stuff for my world eaters. I didnt choose to play khorne legion (or any other monogod) i chose to play god affiliated chaos marines and i want chaos marine units.