Why can’t the joker be played by a black person?

He could be, and I wouldn't be surprised if in ten years time he will be. Right now they're working on a movie where superman is played by a black actor and I bet after that casting will be going to a lot of different races even if the characters have only been white. Also worth noting is that Joker has been played by a black man before in the 2000s cartoon series The Batman and again in the animated movie Batman vs Dracula. 

Because it would be ghetto.