As a land of immigrants (JFK's description), ethnic humour has beeen around in the US since time immemorial. There were jokes about kikes, coons, hillbillies (ENGLISH), dagos, spics, greasers, krauts, scanadihoovians, micks, polacks, bohunks, hunkies, cannucks, etc., etc. They usually came and went in phases such as the knock-knock or moron jokes, all except the POLACK JOKE, which has displayed unusual durability. Why didn't the public tire of them after a year or two? Some say this had to do with the Polish communist government's 1968 anti-Semitic purge which sent thousands of Jews leaving the country or losing their fat-cat jobs. Unable to get back at Warsaw, Jews in the American entertainment industry turned their venom on the msot conveneint ersatz victims -- Polish Americans. Mostly these were anti-blue-collar jokes often involving the allegedly Polish three Bs: beer, bowlinng and bingo, which had nothing to do with Poland.
One went: How can you tell the bride and groom at a Polish (American) wedding? They're the only ones wearing clean bowling shirts.
These joeks were spewed year and year by Rowan adn Martin, Carol Bruentt and other TV shows, stand-up comics and other entertainers, thereby artificially perpetuating them long beyond the normal longevity of your average here-today gone-tomorrow ethnic humour.