Its due to wrestling, this is not boxing!
All you need is a freak like hong man choi to get some form of top game and he will dominate guys like Cain and JDS via unstoppable takedowns and lay n pray.
So JDS and Cain arent really the best, some freak in size is.
(In boxing they separate the giant man from the talented HW boxer many times, but thats where the Giant mans greatest strength is, the clinch, he is slow but he is strong as hell and heavy as hell, so he can take you down, lay on you and squish you like a bug. (at the very least a giant man has a much better chance of winning a fight under MMA rules than under Boxing or even K1 rules.
The UFC dosnt want some huge dude to dominate via boring lay n pray. That would be bad for business.
Even if Hong Man Choi developed some kind of top game he still get smashed by every HW on the roster. Everyone. Akebono lost to Royce Gracie while in top position. I'm sorry but everythimg you said is wrong.