The answer is even easier.
The woman has zero leverage. She won the belt without beating anyone who held a belt and has never defended the belt. She also has very few fans.
If they strip Bisping he'll kick up a fuss, call the next "champion" a "paper champion" and have half of the fanbase agree with him (mostly because he'll be right).
Ditto for MM. You're going to strip the guy who has dominated the division for a record number of title defenses? And give the belt to whom? Will anyone actually consider this new champion the true champion? Doubtful.
But you take the belt from GDR who, like any champion who receives a belt after a random win against a random opponent, is in essence a paper champion herself, and give it to Cyborg, are people going to complain about that? Are people going to dismiss Cyborg as the legit champion because she never had to go through the woman who was gifted a belt out of nowhere?
God, no. The division moves on.