I'm kind of questioning about this too, and I guess part of the trouble with this kind of pics is that they come out so cold. I mean, a naked woman in a cage has a side of humiliation too, of reducing her to flesh and carnality - imagine it in real life! - but that's a hot kind of picture and many women are instantly turned on by this kind of brute helplessness, while a faceless guy presenting his weighed down cock doesn't look hot that way at all.
Maybe it's the fact that the slave presenting a camera shot of his/her own genitals is not one you feel acttively doing anything, except the pic, while a hogtied or caged woman is, you feel, struggling or moving in this space where she's been handed over to the Dom/me, so there's much more of a feel of a relation even if we don't see the other. Just my thoughts...