The reason Ash is always so weak is because they keep 'resetting' him every time he travels to a new region. They do this so that the show can still cater to kids. If they had Ash improve continuously throughout his travels without resetting, then the show's ratings would probably decline and eventually be canceled. (Of course, this is also done to throw off long-time fans who have been following the show for many years.)
If they want to reset have the new girl in Ashs life be the dolt. Its dumb to watch Ash not even know the basics of catching a Pokemon or what type matches what... let the new characters be the ones to teach the new audience and have Ash be the mentor for once. He doesnt have to use Pikachu all the time and Pikachu doesnt have to win all the time either. Theres plenty of ways to go about this series without making Ash look like a idiot over and over again.
Ash leaves his old Pokemon with Professor Oak after each season, except for the Johto Journeys where at the beginning he uses Charizard against a Chikorita. There would be no point to have Ash destroy every Pokemon in sight. Struggles = Character development.It's because, like his game counterpart Red, he has not mastered the godly evolution tool known as the Thunderstone.
Oh, and the whole ditching his whole party and starting over thing. And trying to appeal to the kiddies. Best to think of it as there being multiple parallel universes, all with a trainer named Ash, and best not to think of each series having much continuity from the prior ones.