IT isn't cool enough for girls
Purely empirical evidence tells me that it's contact with other girls at school. Gender culture is insidious. I was doing my part at a high school job fair promoting IT career paths a short while back. The only girls who stopped to chat were classmates of my son who I already knew, and they just wanted to say hi. The only takers for even discussing what the possibilities for study and jobs were, were a few guys who were already interested in computing. IT is not even cool amongst guys, let alone girls.