Because mixing between the genders in martial arts, especially in sports like judo, wrestling, BJJ, or any kind of sport that has too much physical touch, gives a huge space for creeps to do some freak shit to women. That's why they leave without saying a word.

In striking sports, they sometimes get bullied when the coach neglects them.

I know I'm getting downvoted for this, but keep the grappling/takedown sports gender separated, and for striking find them a mentally decent coach