Performance enhancing drugs play a major factor in MMA. It's not just AAS & GH, but there are a myriad of drugs that enhance aggressiveness, dull pain, improve focus and dramatically increase endurance. Anyone taking a cocktail of these various drugs, will be at a significant advantage over someone who is not.
As I understand it, Pride did not test for anything, so drug use was rampant. The UFC does test & punish anyone who fails. This does not eliminate drug use, but it does minimize it. Some fighters are actually clean. Others come off drugs weeks/months before fights, drastically reducing the affects of the drugs.
It seems pretty obvious that many of the former Pride guys have had a difficult time adjusting to either coming off drugs or not being fully on them come fight time.