I think there's so few dominant women because of two factors.

1) There seems to be some sort of human instinct for being either submissive or dominant. Nobody had been able to nail it exactly in science. But there's plenty of related phenomena in all human societies. This thing with people very often being drawn to strong leaders. The whole celebrity thing. There's a huge amount of historical events which cannot be explained any other way. Most people just seem to be subservient by nature. Nietzsche had a thing or two to say about it. and no philosopher after him has really been able to budge that idea.

Also all monkeys display this kind of behaviour, and we're basically naked apes... so it would be pretty silly to think it didn't apply to us as well.

2) The systematic patriarchal oppression of women. Sure, women could be naturally more inclined toward submissiveness, but that is not my experience. The dominant women I've met in my professional life seem more than anything just frustrated about prevailing male values. I don't think it is dominant men who are responsible for keeping women down.

So basically, there are few doms to begin with, and the few dominant women there are have to fight all of society just to maintain sanity. Joining any kind of fringe group in society, like the BDSM scene, does require that you've got your shit together. I can imagine that a lot of dominant women, just don't like the extra stress. And it's not like it isn't submissive guys falling out of the trees. The few dominant women hardly have to look hard, and certainly doesn't have to search web sites for men. Not in any kind of non-professional agreement.

Because I'm a kinky party promoter, (deka dance.se) I come into contact a lot with dominant women. I'd say that their main annoyance is men's inability to submit like sub women do. And their re-occruing tendency to have a meltdown and flee into freedom. This I put down as 100% culture. Groveling for a woman as a man has some serious cultural hurdles to overcome. Still.