imho, comic companies target the market that they know they will get the best results from, and typically that has been the adolescent boy catagory. Since the stories are written to attract the male in, it might just be viewed as a boy thing when girls are young and then it just falls out of their realm of experience.
Any experiance I have had introducing girls to comics has been fairly positive, my girlfriend enjoys them now 🙂
I'm just not sure a lot of books are written with the interests of girls in mind. This doesnt mean that they can't enjoy them, it just means that the Marvel's and DC's of the world know where their cake comes from. There are just some elements of the conflict, the body images and small things like that which have been specifically designed to attract males for profit.
It'd be nice to see girls get into books, but then, I'd also love to see comics break into a more diverse art. meh