I’m not sure how true this is, but I have read that a lot of the female-versions-of-male-heroes were created as a preemptive copyright defense. (Like, there was a case, it went in a way that surprised the industry, and they reacted almost in a panic to create spider-woman and a bunch of others before someone else did.) And that this occurred at a time when female heroes were basically non existent, which is why the reverse doesn’t seem to exist… so again, it’s the numbers.

Oh wait, I found the Stan Lee quote: http://en.wikipedia.org/wiki/Spider-Woman