>>13219545In the Sinnoh portion of the anime it's implied that the pokemons they don't use anymore are no longer part of the team but they most likely still have them boxed or at the very least James did with his Cacnea. There's really no reason for it other than to try and change it up though honestly.
Why did Ash and Brock lose theirs too? It would be boring if they always had the same mons. Don't try to look to far for a real answer because there really isn't one, It's very conthrived.
