There have been a whole lot of wrestlers trying to make the switch over to MMA. They basically have been practicing one aspect of MMA their entire lives. The thing is, many of the best ones still don't cut it. With such a high number of guys trying to make the switch, it's not surprising they are at the top right now. That will start to change too. Guys (at least in theory) should start coming up more well rounded and really good everywhere. You have guys like Roshalt and Roller who were really high level wrestlers that couldn't get close to the top. Then you have guys like Maynard, King Mo, Lindland, Munoz, Phil Davis, and Joe Warren - who are good, but who were/will never be UFC champs.
It takes a special combination that is hard to pinpoint. There are guys like Aldo, BJ Penn, Anderson, and Machida for example, who weren't wrestlers from the start, and have beat the crap out of many of the great wrestlers they have faced.