Why did we start using "Ms" instead of calling all adult women "Mrs"?

[Telemark]

Here’s a recent thread that goes into the history and common adoption of the term.

Why Ms Clinton instead of Mrs Clinton?

Politics aside, why the tendency to write it (and say it, as far as I know) Ms instead of Mrs? I routinely see it used either way. For instance, “Kearson: Why do you think Ms. Clinton is unelectable?” https://boards.straightdope.com/sdmb/newreply.php?do=newreply&p=21911669 Did she get divorced and I missed it, or is the prefix no longer a determiner of marital status? Upon rereading the question before I post it, or is it politics?