Because women are a weird breed, that's why.
But in all seriousness, I think the reason why there are generally fewer women doing photography on a "hard core" basis, is that; While it is a typically creative and artistic profession, you need to master a certain amount of logic and technique.
Photography can be very technical, especially when you get past the "pretty flowers and dogs" stage, when you start mixing in ligh theory, chemical theory, zone system and (dare I say) technical gadgets, women are put off.
It's not that they are less intelligent, it's just that IMO, they want to focus on the art aspect, that may also be why pottery, painting and drawing, and sculpturing, is more popular as art directions.
But, they are still pretty weird though.....