Because there are no women for them to rape. I don't wanna be that guy, but if it comes down to raping a man vs. raping a woman, the thinking is that a woman won't be able to put up as much of a fight.
Then again, growing up I would constantly get beaten up and mugged in my neighborhood. This never happened to my sisters. So why was I preyed upon, at least in that way, but not my sisters? I'm not saying my sisters SHOULD have been raped, but if the thinking is "Women are more often victims of violence because they're the weaker sex," why didn't they get attacked but I did?
Is it "Women get raped, men get jumped?" Because even still, my sisters didn't get their "fair" amount of assault. And I was just as careful as they were.