It's isn't because they are prejudiced, it's just if some "felon" or "criminal" gets pulled over on the side of the interstate at 11-12-1 at night, and sees a normal sized woman walk up to his window, he most likely thinks he can "handle" her.
I'm not against woman LEO's. I actually think they work well with the other LEO's in a group. Woman just handle some situations differently than men would. They have their role in certain area's as a LEO.