The writers kept wanting Ash to start low level at the start of each season.
It was an issue they hit first with Johto when Ash kept his Kanto team and Charizard completely sweeps Casey's noob team because of course it should - so they had to have Ash have reasons to drop his Kanto roster fairly quick to make room for Johto mons
Then going into Hoenn they started the tradition of Ash boxing everyone except Pikachu - which means Pika has to keep being wildly inconsistent.
