Maybe it's just the particular zines you're reading or blogs you're finding. I mean, there are many reasons that women do zines which are associated with things like feminism and crafting (that have historically been associated with women and women's movements), but - again - I think it depends on which corners of zine cultures/communities you are looking at. Just because they are not on this site does not mean they aren't out there and there are many, many men (as well as genderqueer, transgender, etc...) people who make zines.
To extrapolate a bit more on the feminism drive that Betty mentioned: zines have always been an alternative form of media for those whose voices aren't otherwise getting heard. From what I've read, many, many female-identified zinesters see themselves as poorly represented or even completely absent from mainstream media portrayals. (Cosmo isn't going to do a feature on grrrlVIRUS any time soon. And even if it does, it'll be right back to how to please your man/lose weight/get trendy in the very same issue.) It's the same reason queers are disproportionately represented amongst zinesters: when you're part of the Other, you've got to make your own media. So why all the women? Because the mainstream media - and I'm looking at you in particular, advertising industry and capitalist models around childhood - is still treating women like they're idiots.