I have a theory that it comes to the fact the great majority of these fighters are Americans and wrestling is widely train in America since young age in many schools and continuing through university.
I guess this is something that is quite overlooked. As you can train wrestling in college in the US, there are probably a lot more people with a wrestling background in the US than with other disciplines. So also the pool with fighters having a wrestling background is much bigger, so it's also more likely to find good fighters with a wrestling background. On the other hand, most of the Brazilian champs probably never came in contact with wrestling before starting their MMA career, yet they obviously do pretty well.
So it's probably rather a question of availability concerning the base of a fighter. Later it's really about learning enough of ALL aspects of MMA and become a MIXED martial artist.