The gender imbalance is systemic, and not at all that hard to explain. In the early 90s, as home PCs were becoming ubiquitous, they were in part marketed in the same vein as any other masculine hobby -- auto repair, diy tinkering, etc. The "titans" of early tech companies were men -- not because women weren't interested or weren't smart enough -- but because socially, the gender stereotypes were such that women need not concern themselves (i.e. "Women need not apply").
Ultimately, the same systemic sexism that refused women the right to vote until they pushed hard enough, or refused women equal pay, is the same systemic sexism that keeps women away from CS.
Someone else commented that perhaps it takes a certain "grit" (paraphrased) to maintain interest in CS. I think they're so wrong they're right: It takes a certain "grit" to continue in such a "programmer-bro" culture, where there are few female role models.