1. The benefit of a larger talent pool to recruit from is easy to measure, and the drawbacks (having to fire a large number of them and increased contraband in the prison from said compromised COs) are harder to measure.
2. It's politically unpalatable to segregate by gender.
The vast majority of female COs wash out within their first year; either they can't take the pressure, or they start showing favoritism toward the more charismatic inmates. The reason is pretty simple - women are fucked with more than men. For whatever reason, when a male CO gigs an inmate, the inmate knows that it's The Man keeping him down and not the man. In contrast, when a female CO gigs an inmate, the inmate tries harder to get out of it because he perceives weakness. As a result, more female COs crack.
This actually happens with all staff, not just guards. Nurses, instructors, whatever - all of them get routinely compromised. It's a very, very difficult problem to hire people for relatively low wages, stick them with people who have nothing better to do, and expect everyone to remain professional.
The optimal male CO is an unthinking robot. "Sorry, you violated Rule 131. I have to write you up. No exceptions." In contrast, the optimal female CO is a "crazy dyke psycho bitch" who takes pleasure (or appears to take pleasure) in making prisoners' lives hell. They won't respect a woman who acts otherwise. Again, this is because inmates respect The Man but take it personally when a woman tells them No. So, the woman has to show that she will not ever let them off because she enjoys making them suffer.
Source: Girlfriend worked corrections for a while. Her "prison attitude" is hilarious.
"I prefer corrections to bartending. When customers at the bar sexually harass / assault you, you have to smile and take it. When inmates sexually harass you, you put them in solitary and forget to refill their pain med prescription. Whoops."