Well, I think it has to do with the fact that most comics are drawn by guys. A guy into femdom knows what he wants done to him. A guy into femsub knows what he wants to do to someone. Drawing a comic where something happens to a character is much more like doing something -to- someone than having it done to you. So a guy into femsub is going to find it more satisfying to draw the femsub comic than a guy who is into femdom is going to find it to draw the femdom comic.
At least, that's my best guess.
Smack