Why women don't blog: because people say things like "why don't hot chicks blog?"
Seriously, I don't want to be all humorless-feminist about this, but there are plenty of good reasons for women to not have an obvious public presence on the internet. This Crooked Timber discussion has some good points about why it seems like there are fewer female bloggers, expecially academic types.
Many people think I'm nuts to have a blog, especially under my real name. It's possible that it's hurt me in the competition for jobs, and I'm okay with that, but it's a lot harder to get an academic job than a lawyering one, and people hunting for those rare tenure track positions are understandably risk averse. Couple that with the (unfortunate but still strong) subconscious tendency to categorize opinionated, argumentative men as assertive and women with the same traits as bitchy, and you end up with a tidy incentive not to blog, or at least not under your own name. Why engage in an activity that could get you branded as "uncollegial?"
Then there's the attention. Women online still get a lot of attention from strange men, expecially if we write about something slightly risque. Many women aren't comfortable with getting come-on emails and compliments/speculations about their appearance from random men. This will happen even if you post anonymously, if your handle is recognizably female. This sort of interaction could drive women to assert their online identities in safe, female dominated spaces like Livejournal, iVillage, or feminist blogrings instead of what most people reading this will think of as the blogosphere.
To be a female blogger in the war/politics blogosphere, you need a thick skin with regard to criticism and sexism. Not all women are willing to put up with the incivility that sometimes dominates blog discourse. So we create our own corners of the blogosphere, our own friends-only sites, our own communities. Saying there aren't enough female bloggers is easy. Finding them, and finding out why you didn't see them before, is more of a personal challenge.