They might not want to get pulled into the wonderful world of women getting beat on by men in video games.
Granted, there's always been girls in things like Street Fighter and etc but maybe they're looking to avoid creating nice female images (hopefully something different from sword-tits or revolver-ass paintbucket stereotypes) and then filling them full of bullets and various melee strikes until they explode into parts or fall over and bleed out. I know TF2 is a bit more comic than most they're-all-wearing-power-suits-anyway-what-does-gender-matter FPS games but still, it might just be the design they want to go with.
I'd also argue it lets female gamers go unnoticed, though I doubt the good/serious ones care about that kind of thing. MMOs, I've found, are always odd for the fact that when you run into a female toon you usually think there's a 50%+ chance that it's a guy, which is sometimes creepy and sometimes doesn't matter at all. With all toons male though I just toss the issue of wondering out the window and treat all players the same, which I'll admit I don't do otherwise.