The Parameters are just different. Just because someone is good at football doesn't mean they'll be good at Basketball. People have different builds and different things they excel at. Maybe somebody like Floyd Mayweather has phenomenal boxing skills but maybe his takedown defense would never get to the levels of say GSP. It's about combining the arts, not about one specific art. Even the fighters that are way ahead of their time in terms of one particular area of their skill set would get their ass handed to them if they didn't have a bit of the other disciplines under their belt.
Conversely maybe somebodies MMA Skills may be well rounded but naturally people that spend their life focusing on just one aspect on mma and then limiting a fighter who has been cross training and saying "okay you can't kick now" is going to give the advantage to the guy who has strictly been working on his head movement and hands.
Put some guy who has been training one specific element and throw them in with someone who can do it all and the rules favor them, guess who is going to win? Take someone who has been training multiple arts for 20 years versus a guy who has been training 20 years specifically on one art and you can only use that art, guess who is likely to win?
More often than not it's not hard to predict what would happen given those circumstances in anything. The same would apply to people who study different things. It's just common sense, I honestly don't even know why this is a topic people discuss.