Some people can roll their tongues, and some people can not. I can't, personally. These are differences in people, but there's not a big market for humor about this. Not to mention there are no jokes about people with wrists that have bones protruding from them like I do. When you are thinking of funny jokes to tell people(I assume, I don't have a sense of humor) you don't think "hey, I think some tongue rolling humor would be great". Because you really don't think of it as a big divider of the human race. Maybe it's a neat trivia piece to know about someone, but not a big arbitrator of identity.
But if we thought that all people who can't roll their tongues were mean loud people who steal cabbages and rutabagas, we'd probably be rolling in the aisles with all the non tongue rolling jokes..