Why? Well, like I said, there are more questions that have to be addressed before we know why, but I'm betting that a big part of the problem is this:
Canizares of MIT said that he saw one area where he feared research universities were becoming bad employers for both male and female scientists. He noted that with more departments expecting multiple years as a postdoc before becoming an assistant professor, new Ph.D.'s are increasingly facing the prospect of longer and longer time periods "before knowing where [they] are going to end up," and even more years before knowing if they will earn tenure.
"We are making the career less attractive for both men and women, but women have extra factors that make that time scale particularly unattractive," he said.
I can say this from experience: without question it's hard on men with children, and I'm sure it's worse in general for women who are trying to balance career and family. Biology is one of the worst fields in terms of time from PhD to that first faculty job and time to tenure, and, not surprisingly, biology was the worst field in the study in terms of women failing to apply for tenure-track jobs: plenty of women get biology PhDs, but few apply for academic jobs.
Here's my experience: I've been a postdoc three years, after almost 6 years in grad school. I have three kids, one who just finished 4th grade. I'm in my early 30's. And in a year or two, who knows where we'll live? At my age and with my family size, I should be settling down somewhere, saving for my kids' college educations, and basically establishing myself somewhere in the middle-class American Dream.
The reality is that, after 10 years of training and research experience, I have no long-term (not to mention medium-term) job security, no retirement benefits, my health insurance is mediocre, and one significant health crisis could easily send us into a financial tailspin.
The bottom line is this: a career in academic science, especially biology, demands a lot of you in terms of training, skill, time, and dedication, and the rewards are uncertain and in any case a long way off. Obviously doing science is great, which is why a lot of people still go into the career, yet perhaps we're luring in fresh undergraduate recruits with a little bit of false advertising: you go in thinking what could be better than having the same kind of job Einstein had, and then, 12 years later, it dawns on you that it's actually kind of hard to stake your claim to a corner of the scientific landscape that shows potential for paradigm-shifting discoveries. You can go through years of training, letting the opportunity costs add up, and wind up working on research problems that are interesting, but not enough to keep away the doubts about your career choice and the opportunities you gave up to pursue science.
I've seen it happen repeatedly. Personally, I love what I do and, having been born when my father was in grad school, staying on the move until he got his current academic job when I was 12 years old, I came into this job with my eyes open.
So why are women dropping out of academic science careers? There could be more to it, but surely the fact that we keep stringing people along for too many years with the tantalizing but uncertain promise of a lab of one's own plays some role.