Why are most offensive lineman white and defensive lineman black?

[Jack of Arcades]

I mean when you think of the top young left tackles, only one of them is white (Joe Thomas). Well, Jake Long, too. But you have D'Brickishaw, Marcus McNeil, Jammal Brown, etc. The dominant LTs in the past years have been black: Willie Roaf, Walter Jones, Orlando Pace, Bryant McKinnie.