The problem with racist humor is how repetitive and dull it generally is.
Wow, you called a black person one dumb chicken eater, how completely unpredictable, Mr Hitler.
Add up the fact that mostly legit racists take themselves way too seriously, and you have one endless chan of one-side jokes. It`s always that one target, and all you have to do is repeat the thing you already said thousand of times. So, in short, they think of themselves so big and compromised to the cause that they kill the humor in the process. In fact it was never their intention to make people laugh, only to spread bullshit agenda in comical ways.
It does not mean, of course, that stereotypes jokes are doomed from start, or that people will ban every single stereotype, like some /pol/tards fear mongers seems to believe
The key word is intention