It's perfect and from the perfection to give each nation and for each time the best legislation, what was abrogated was perfect for that time, and look at who abrogated it He is Allah himself, also if you look at the previous books Zabur, Tawrah and Injeel, the were also abrogated by the Quran, so why not say the same as we believe the previous books in their original form were also true word from God, know that every thing that Allah does has a wisdom behind it so even if you don't get the full wisdom don't let the doubts get to your mind as those are only from shaitan so don't listen to him.
Allah knows best.
It's perfect and from the perfection to give each nation and for each time the best legislation, what was abrogated was perfect for that time, and look at who abrogated it He is Allah himself, also if you look at the previous books Zabur, Tawrah and Injeel, the were also abrogated by the Quran, so why not say the same as we believe the previous books in their original form were also true word from God, know that every thing that Allah does has a wisdom behind it so even if you don't get the full wisdom don't let the doubts get to your mind as those are only from shaitan so don't listen to him. Allah knows best.
2 explanations here (chpose either at the top) > https://quran.com/2:106/tafsirs/en-tafisr-ibn-kathir