We knew at some point we weren't going to be able to indefinitely keep supporting all of the Pokemon, and we just found that Sword and Shield would probably be a good point to go back and reevaluate what would be the best selection of Pokemon that appeal to the widest audience while keeping into consideration the balance of the battle system. It isn't just going to be all-new Pokemon in the Galar region Pokedex; there's still going to be a lot of favorites that fans will be able to bring over that they've adventured with previously. But yeah, it was pretty much just balancing and getting this optimal selection of Pokemon for the adventure we wanted to provide.
Cit.