I've been thinking. Part of the reason they're not letting you bring in outside Pokemon is because they want to give the new region a sense of identity. I can understand that. Because that's how the games have generally always worked prior to the post-game. You'd play through the game with various implicit or explicit limits on pokemon selection and get a feel for the identity of the region and how the Pokemon play into that identity. Then once the main adventure is complete, the world opens up. The games generally try to give you the feeling that your current region is just one part of a larger world, ever since Gen 2 when you were literally allowed to explore the region from the first game. The national dex and the ability to bring in Pokemon with identities tied to other regions played into that.

By comparison, Galar is apparently a land of xenophobia.