Without getting tooooo theoretical, I think the answer lies in the way people are apt to squick out over a BDSM image (or story). There are lots of ways to engage with the characters in an image, but a common source of anxiety/squick/disgust/insecurity is to say: oooo, I would not want to be him / her, get me out of this fantasy.
In BDSM, that gets more intense around with images of bottoms. You can see a top doing something you wouldn’t be into, but you say “well, if that was me I wouldn’t be doing that.” That’s harder to say for an image of a bottom, because they ostensibly aren’t in control of what’s going on.
Another very easy dodge is to say: well, that looks unpleasant, but it’s happening to a girl/boy, and I’m not one of those, so everything’s cool. I think this response is sexist and weirdly insecure, but I’m guessing it’s real enough to have a big impact on the way people arrange their fantasies.
TLDR: It’s hard to find pan-sub images because people are afraid of scaring off the gender essentialists.
Oh, now that’s interesting!