Because then they wouldn't need to constantly change Ash to fit whatever plot they want to do like regressing his skill in BW. Using the same character for every new season means that character will bring a bunch of baggage and people will take issue when they change them to act in a way that makes no sense to what is established before.Thing is, if all the protagonists were Ash with a new look, why replace Ash in the first place?
It's basically what's happening to shows like The Simpsons, where it has gone on far too long to the point the writers don't know what to do with the characters, so they start making them act out of character to fit whatever plot they want for each episode. Unlike the pokemon anime though, The Simpsons should have ended ages ago instead of having new main characters.