That's not entirely true, as there are plenty of women in the life sciences (unless we were to spark an internecine war within STEM by claiming that bio and medicine aren't the hard sciences). Anecdata seems to show that there are also more women in physics, math, and other engineering such as civil or even mechanical and electrical than there are in CS.
One possible cause may be the traditional nerd stigma that afflicts CS. In recent years it's broadened up, though ironically now there's a small fratty brogrammer subculture in CS. (Though not in academia, I feel, but in the startup industry that follows.)
I know there's more, but the numbers are still far from even in many hard science & engineering degrees. I started out in Civil Engineering and even there, the ratio of guys to girls was scarcely better than when I transferred to Computer Science (this was in the few degree specific courses I took as well).
I do consider Bio and Medicine to be hard sciences and you are correct the gender imbalance is significantly less than some other science fields. However, it seems to be more acceptable in American Culture for parents to accept their daughters going into those types of degrees perhaps due to the relation to traditional career paths women have gone into (such as nursing). If that's true, it's rather sad and hopefully such preconceived notions die out in the near future.
I'm curious though as to how many female students end up going into Computer Science or Engineering because a family member or parent was in one of those fields. I have a few female friends either pursuing or have obtained a degree in Computer Science and each had at least one parent that was in a related field.
One possible cause may be the traditional nerd stigma that afflicts CS. In recent years it's broadened up, though ironically now there's a small fratty brogrammer subculture in CS. (Though not in academia, I feel, but in the startup industry that follows.)