Because storm troopers and the Imperials were based on Nazi Germany and the Imperials are supposed to be racist white supremacists. Having a black storm trooper is like having a short storm trooper. Did you ever see a non-white person in any Imperial role in any of the original trilogy movies?
Although they already kind of shot themselves in the foot by making all the clones Latino in the prequel trilogy.
Although they already kind of shot themselves in the foot by making all the clones Latino in the prequel trilogy.