[Episode Discussion] Why Ash hasn't actually brought any of his reserved Pokémon back as of the Best Wishes! (sub)/Black & White (dub) series?

'Sup, guys. Back in the original subbed version of Best Wishes! series (known in the dubbed version as the Black & White series), Ash has never actually brought any of his reserved Sinnoh, Unova, Kalos and Alola Pokémon from at Professor Oak's Laboratory and from at Profs. Kukui and Burnet's home, and he hasn't actually brought any of his reserved Pokémon from Professor Oak's Laboratory or in care by other Trainers, including Squirtle and Primeape, for a long time either. Now I wonder why you think, and do you think that he will make his comeback to bring any of them back? And how about his released Greninja, too? Why? Please let me know, thanks. :)

I have the same thought

i guess he just wants to start fresh in a new region? if i had the chance to catch all new kinds of pokemon, i'd for sure free up as much slot space to accomodate them.
^{although i'm sure there must be a canon explanation that i missed bcs i don't watch the anime much anymore...}