There is something just innately more disturbing, initially, to viewing someone on the ground being kicked by someone standing up. I think it is because it is something everyone can do to some degree, where as kicks to the head, etc, while on the feet take serious skill to do mechanically well and land. I love MMA, but came to it just after the pride heyday, but through finding Fedor youtube videos(his beating of Hong Man Choi especially), and even knowing what I know now, can't shake the feeling that soccer kicks to the guy on the ground is brutal. Yet I know mechanically speaking they are no more powerful, less than even, than spinning wheel kicks and hook kicks. This leads me to believe that they won't be allowed for a long time, if ever, because we as a society are just not ready. There is a reason they were left out of the unified rules, and while we might not like it, it is perfectly understandable, most people abhor violence in any form - that includes combat sports - and the image of someone kicking someone on the ground is one of the most violent images, that doesn't include weapons or deaths that a normal person can comprehend. Perhaps sometime in the future, public perception will change, but it won't change because a few million people who follow a niche sport say it should, at least not in the foreseeable future.