Because "he" and "she" have commonly understood meanings (even if currently the words mean a few different things, to different people, in different contexts). But there's no broad understanding of what it means to be any of the neopronouns. But also, the further we get away from pronouns referring to any material reality or common understanding, then yes all pronouns become preferences, whether "she" or ":]" or "xir."

Can you help me understand why you think pronouns, particularly any of the neos, aren't a matter of preference?