I'm thinking it was something to do with her voice actress. Ai Maeda was a fairly popular name at that time, so they could probably only get her for a certain nuraber of episodes. ReminRAB me of how they had to recast Joseph on King of the Hill.
That's because Masami Kikuchi is (or was at the time) a bigger, more expensive name. It's why they had to limit Jou's role; it's expensive to get his voice actor.