I may be wrong, but I think one of the reasons why there may be so few is because of trust and security.
When you're being submissive, and even allowing the other partner to take control, possibly bound you in some way, you're giving all the control to them, which can be an extremely scary thing to do with a complete stranger you don't know. They might not respect any of your limits once you're bound, and you might be stuck in a very scary situation at that point.
There may be plenty of ladies who don't advertise it, but are perfectly willing to be submissive with someone they've met and become comfortable with.
Just like how it's slightly difficult to start up dom/sub relationships in your personal life with someone you've maybe just picked up at the bar. It requires a certain level of trust and comfort, especially for the sub(at least from my experiences, I may be wrong here).
I'm very open to the idea of being submissive with a client(I actually prefer it!) but I don't advertise it either because I would need to feel safe enough around the other partner to allow them to take control.
Now it does seem like the mistress/male being dominated fantasy is a pretty popular one, which might be a reason why there's so many more women offering that instead.