Now consider hits to the back of the head. If an MMA fighter exposes the back of his head to you, the quickest way to end it would be a palm strike to the base of the skull: lights out. But because that's illegal fighters will be a lot more likely to go for something like a rear naked choke, taking the fight to the ground and artificially inflating the value of grappling.
And that's just three of the many illegal moves.