I think it's because receiving oral sex for a woman has some emotional issues tied to it. For me, in order to receive and enjoy, I have to be very comfortable with my partner and trusting. If we don't have that, I'm not gonna let him near my crotch. As for going down on him...well, I love it, and quite honestly a guy is hardpressed to keep me away from his crotch.
It stands alone and makes good foreplay.
