In our home dojo, the ratio is about 50/50 - with the dojo head being a woman herself.
In our parent organization, the ration drops dramatically - although we women still have a fair number of extremely talented women to look to for constant inspiration.
As for why - I'm not sure! As a woman, I was attracted to Aikido because it has a natural grace and flow to it. Frankly, it allows women to draw some of our major natural strengths - our hips, our flow, our grace.