What makes MMA so difficult that there is only one truly dominant fighter?

Another shocking loss this weekend with GSP being koed by Serra. It seems everytime there is a fighter who looks unbeatable he loses. GSP and Gomi I thought were nearly unbeatable. Now fedor is the only fighter I can think of who has had a near perfect run. He did have the one loss due to a cut but other than that he has been perfect. In boxing you see fighters go undefeated for long stretches of time which is probably due to more margin for error. In MMA everyone losses. Why is it so difficult to win consistenly in MMA?