Probably a multitude of reasons, really. Racism is definitely on the list of reasons why black wrestlers haven't had an equal cup of success in wrestling. I think the racism extends just as much to the fans as it does/did to promoters and workers alike. The ladder of superstardom is not as easily attained in professional wrestling as it is in other sports, like the aforementioned baseball, football, and basketball.
I think more racism from inside the business than from the fans.
Promoters/bookers/wrestlers play to the racism of the fans. They also play to their own racism looking for reasons to avoid developing/pushing black wrestlers.
If it wasn't proven before, Rock pretty much ended the myth that wrestling fans all of the country wouldn't get behind a black wrestler.
But I think long before that the myth should have been dead. Blacks in other forms of entertainment have been wildly "cross over" popular for decades. In turn, taking the fake-sport to real-sport comp, black athletes have done big business in sports through the years.
Wrestling has always been like the mafia: a closed business where the "us" is protected.