I think it is a combination of there being fewer girls inclined to it, and that society kind of discourages us, in that CS is perceived to be done by only males.
I go to an all-girls school, and 10 people including me do it (year group of about 80) and 4 people do it at a level, which i'd imagine is a higher number of girls than most schools. At all-girls schools, people are more free to express themselves and are more encouraged to study what they want, i know that i would be less comfortable doing CS if i was the only girl in a class full of boys.
There have been plenty of female computer scientists over the years, so it's not like we're incapable of it, but it can be difficult to separate nature and nurture.