Well, I’ll use some rumors I heard to answer this question. Obviously, it’s just what the creators of the anime wanted, but I’ll give you a bit of a guess answer.
So, when the Galar anime came out, there was some speculation about Ash leaving the show, because he beat the Alola league. Maybe this has something to do with him traveling region to region as a researcher. Though this may sound a bit crazy... it’s a theory.
Another thing is that the creators wanted to mix it up a bit. Every single Pokémon anime saga, Ash starts in a new region and beats gym leaders to claim the title of the Pokémon League Champion. You also see more detail on Ash and Pikachu in this anime. It could just be focusing more on where Ash and Pikachu are from, and Ash revisiting Kanto and taking on a different type of journey.
So really, there isn’t a clear answer. I just gave a couple theories as to what might be happening.
Hope this helps! :)