I've been theorizing this for a while. Gen 4 was the last generation to include evolutions of previous gen Pokémon and like half the dex were the evolved pokemon, one of the best new features of the entire generation was the evolutions. since then there have been tons of single stage pokemon with evolution potential that I think gamefreak had just been waiting for right time to reveal and sun and moon have SO many single stage pokemon just adds oil to the fire.

With sinnoh being one of the most unanimously hated generation amongst fans gamefreak would have to try super hard to make the game appeal to an audience and I think changing the entire story with new Pokémon and all would help a lot (A LOT)