Explanation: As seen here, racism was rampant amongst the American brass (and rank-and-file, for that matter) during WW2. Despite African-American soldiers having repeatedly proven themselves in prior US wars, there was a strange notion in WW2 that Black people were in some way 'incapable' of combat equal to the White Man(tm).
For that matter, Patton himself, who was not much more than a fascist born on the non-fascist side of the national borders of the period, was deeply racist in private, both against Black people and Jews, but he, at least, knew enough to play the leader to troops under his command instead of attempting to demoralize his own fucking forces.