For the same reason guys don't apply to "America's next top model". There just aren't enough female geeks. Which is a pity, since they are mathematically better tuned.
As for the "mathematically better tuned" comment, I've always disliked the arguments that one sex is better suited for anything, especially when talking about the brain. I think socialization has a lot more to do with brain development then sex, and since we've got hundreds of years of gender roles to overcome before we find out for sure, I think we're better off judging individuals, rather then genders.
As for why there aren't more women in startups, or women geeks for that matter, I return to socialization. Bill Gates made it cool to be a nerd for guys, we still don't have a female in that category. A largely successful role model that makes it ok for women to not be entirerly obsessed with their appearence. Maybe when I get my own start up...