Because it's much easier to get people to use an existing pronoun in a way that historically would have been considered a bit sloppy than it is to get them to adopt a new non-sex gendered but non-neutral coined pronoun. (It's actually been tried in the past.)
This really just a way of rephrasing what the parent comment says. By some reckonings, in English, pronouns are a closed class, and verbs are an open class. Some languages have it the other way around, where you can invent new pronouns but not new verbs (you have to take a noun X and say “do X” or something like that).
Ms. did have its own political angle but but when it did finally become mainstream in the late 60s/early 70s, I'm not sure that there were too many people ready to go to the mat for public honorifics that distinguished whether a woman--and only a woman--was married or not. (And at least some people who probably reflexively opposed the change at first, changed their mind later on.)
But honorifics are tricky and some people feel very strongly about them--e.g. Dr. for someone with a PhD.
