I’ve found it’s because Game Freak wants each Gen to have its own unique feature that sets it apart from other games. If every Pokémon game had Dynamaxing and Max Raid Battles, people would just get sick of the same thing (which is why I think those features may go away). SOS chains and Battle Royales made Alola unique, and those are two things that you can just picture the Gen 7 games when they’re mentioned.
It’s also slight Game Freak logic.
Hope this helps! :)