Apparently, according to the head of animation on an upcoming kids movie, it's because "animating female characters are really, really difficult, because they have to go through these range of emotions, but you have to keep them pretty."
He goes on to complain:
So, having a film with two hero female characters was really tough, and having them both in the scene and look very different if they’re echoing the same expression; that Elsa looking angry looks different from Anna being angry.”
Jesus, two female characters in one movie? Someone get this guy an Oscar or a Purple Heart, already. I can't imagine what a nightmare that must've been.