The poor have always dominated sports...professional and otherwise.
The children of successful classes spend their time in front of computer monitors, playing electronic games.
In addition, blacks are driven to excel at sports. They would be tremendous athletes in any case...but they also have something to prove.
And say what you will about the political incorrectness of this next statement, there is the distinct possibility that it is true. The institution of slavery weeds out the weaklings. To begin with, slave buyers or traders would selectively buy only the stronges slaves available in the west African markets. From the deadly trip across the Atlantic on a crowded slave ship to the terrible physical grind of day-to-day drudgery, slaves lived an existence that cleaned up the gene pool.
Mankind is part of the natural world gentlemen. The fact that dogs and horses can be bred for selective traits, such as size, speed, and intelligence...raises the question of "then why not man?"