This is just of my opinion (#NOH8) but:
This is a question that I've often thought about myself and it is quite a tricky one! Apparently there are more Rap fans that are male than female. Now as a female, and I speak for myself here, I'm far more likely to enjoy and be stimulated by the male voice than the male. I find Chris Brown's voice very sexy, but I also like Jason Derulo's who I find sexy too, but it's lighter more cerebral. Currently there are more female vocalists on the scene, but if you go back a decade or two there were as many male. Maybe they were more pop/jazz oriented though. My collection has nearly all female singers. I don't think that there is too much demand for male singers these days. Pop fans today want to hear that softness in the voice, not the toughness. Things may change, it could be just a trend.