For the same reason not many men become maternity nurses or kindergarten teachers?
Frankly, you couldn't have kept me away from IT if you'd tried. Despite low wages (when I started, at least), poor conditions, working on short-term contracts, being required to educate myself, putting up with being treated like some kind of idiot savant, etc, etc -- still, I wanted it very, very badly.
Now, for people who don't feel that way, we're supposed to engage in this collective hand-wringing exercise: "Oh dear! Not enough wimmin in IT!" If they don't want it, I would seriously question the morality of (so to speak) tricking them into joining the profession on the basis of a collection of half-truths. Either you want to play with the shiny toys or you don't. And if you don't, good luck, because 90% of the time it's not exactly what you'd call fun anyway.
Incidentally, I've been around a bit and I have never, ever witnessed any indication of prejudice against women in IT. Where I've worked with female engineers, they've always been welcomed and treated with respect. Maybe we should just accept what any parent can tell you: there are gender differences, and we need to accept those rather than pretend we're all the same.
/rant. calming down now ;) talk amongst yourselves...