It is because the majority of them was racist? It is because nobody wants to hear someone talks to 4 min? It is because of smartphones? It's the changing of generation? although, i don't even hear old people telling those. As exemple, every joke who started like this: "there was a guy..."; "there was a woman..."; "once a men said to his woman...".
Edit: better example of this type of joke, At this point I'll tell you the joke of a woodchopper from the Middle East who came looking for a job at a lumber camp.The foreman said, "I don't know if this is the kind of job you want; here we chop trees." The woodchopper said, "That's precisely the sort of work I do." The foreman replied, " Okay, here's an axe, let' s see how long it takes you to chop down this tree here." The woodchopper went over to the tree and felled it with one blow. The foreman, amazed, said, " Okay, try that big one over there." The woodchopper went over to the tree-biff, bam-in two strokes the tree was down. "Fantastic!" cried the foreman. " Of course you are hired, but how did you ever learn to chop like that?" " Oh," he replied, "I've had plenty of practice in the Sahara Forest." The foreman thought for a moment. "You mean," he said, "the Sahara Desert." " Oh yes," replied the woodchopper, "it is now!"