>>49962459BECAUSE THEY COULDN'T DITCH PIKACHU
That's the real answer. Just that and nothing else. They couldn't use a different character for each gen because they needed to keep pikachu around, and replacing him with a pikachu clone or even a different pikachu each time was out of the question.
So Ash absolutely had to stay. It was non negotiable. There will never be an episode of the main series without him because he and that specific pikachu are a package deal, as obviously him giving it away is also out of the question.
