I'd say that it is because, just showing legs and butts is just showing parts and more people want to be viewed as an athlete, bodybuilder, weightlifter...whatever they are without being degraded to a piece of meat.
Besides, with a good torso pic, the face is visible and that is very important. I'm here to show that I am a proud gay athlete. You guys can already guess I have a gay ass. But, if you insist, perhaps RealJock can inact a policy like DudesNude did that one or more of your pictures have too be full body, head to toe shots. I'm ok with a nice torso with the face visible, but a full body shot is always a welcome sight, gives you the whole big picture. But, aha, I've found them few and far between: guys who won't show you their ass and legs on cam if you ask politely.