I have a suspicion that women who get into tech tend to be more self-assured than the average individual (by virtue of the fact that they're willing to fight what looks like an uphill battle to get into the field). And that those people are also more likely to look around for a better opportunity and not be afraid to grab it. Which would make them inherently difficult to retain.
(I also have a feeling that talking about women in tech in any kind of generality is a good way to get pitchforked and set on fire, so I hope I haven't said anything out of line.)