Football, more than the other sports, is so racist precisely because it recreates the imagery of slavery itself.
First there's the combine, with all the measuring of black bodies, rank speculation on their monetary worth, and the leering, pseudo-sexual gaze of older, white, impotent men (coaches, fans, scouts, writers).
Then there's the practice field and the whole training camp. You've got black men literally running around in the heat all day, breaking their backs, putting their bodies on the line for white plutocrats. You've got a gang of overseers: head coaches, assistants, etc. all dogging these athletes. Worst of all are the team owners, who stalk the sidelines silently, observing their "property".
Adrian Peterson wasn't right when he said NFL players are slaves, but was definitely on to something.