Why is the southern accent the only one you ever see anyone try to write? The first written southern accent I ran across was for the character Duke in the novel M*A*S*H (a character who was in the movie but not the TV show) and it confused the heck out of my 13 year old self. Duke kept using "y'all" when referring to one person, for example. Meanwhile, Hawkeye never spoke with an accent, despite being from Maine. Just a pet peeve of mine...The problem with the Maine accent is that it's almost impossible to transcribe short of using IPA. Wiley Miller tries it, from time to time, but it undercuts the humor every time he does so. First you have to translate; then you have to try to get the joke; and by the time you've gotten that far, the whole thing's usually gone pretty stale.
As for other regional accents: watch Drop Dead Gorgeous sometime, which for my money is one of the funniest comedies ever made. There is a distinctive Minnesota accent that the makers of this film nailed absolutely dead to rights, and the proof is that most native Minnesotans, when shown this film, respond by saying, "That's not funny. We don't really talk like that."
And they will say it in the accent from the movie!
(By the way, when #3 Daughter is in the right mood she can start riffing in Minnesotan and it's just a hoot, doncha know.)
There are a number of problems with doing dialect humor. The first, come to think of it, is one I've caved-in to by reflex; we're talking about dialect, not humor, but the use of intrusive dialect almost automatically identifies the writing as humor. For that you can blame Mark Twain and Bret Harte, among others.
A second is that unlike the U.K., where regional dialects seem to have been fixed by royal proclamation sometime around 1066 A.D., American regional dialects come and go, sometimes quite rapidly. Washington Irving wrote a lot of humor in a Dutch-based New York dialect that's nearly incomprehensible today. The Brooklyn accent had its hey-day in the middle of the last century and by the 1960s was as stale as jokes about da Dodgers, dose bums, as youse guys well know. (By the way, one of my pet peeves was the TV show Happy Days, and moreso its spinoff, Laverne & Shirley, in which they didn't even try to get the Milwaukee accent right, but instead opted for a sort of sloppy New Joisey/Brooklyn dialect.) Valley Girl came and went so fast we barely had time to like, notice, totally, for sure. And of course, there is absolutely nothing older than slightly out-of-date urban black slang. South Park once did a great bit in which Chef (Isaac Hayes) explains to Mr. Garrison that the entire point of urban black slang is to change it faster than white people can keep up. Solid. Right on, brother.
There is also the matter of evolving sensitivity, or perhaps simply fear. I defy anyone to watch or listen to one of those old "Jack Benny and Rochester" sketches now without wincing. God Forbid anyone but Billy Crystal, Jerry Seinfeld, or Adam Sandler should ever use the New York Jewish accent, oi vey! Jamaican can only be used in special cases, mon, and ever since Star Trek, ye cannah use the Scots accent f'r ennything but parody, captain. Comic Hispanic came and went around 1960 with Ricky Ricardo and Bill "Jose Jiminez" Dana, and came and went again around 1970 with Cheech & Chong, but these days only Cheech Marin and George Lopez dare to use it in public without fear of getting their lips sliced off, and Marin rarely does anymore.
Which leaves us with our old standby, the Comic Southern accent. (Duke in M*A*S*H was a comic relief character, after all.) It remains because it's easy to write, easy to understand, instantly conveys a whole passel of collateral information about them folks what speak that there dialect, and not the least, because southern white Christians are about the only people left who it's still safe to mock. Not that there actually is one universal southern accent, of course: Georgians speak differently from Tennesseeans, who speak differently from Kentuckians, who speak differently from Texans, who have at least four distinct regional sub-accents that I've identified so far. And of course, Louisianians speak vastly differently from all of the above.
But thanks to 150 years of East Coast publishers printing "regional" humor, 80 years of talkies enshrining the long tall Texan and his rustic comic sidekick, and 50 years of The Beverly Hillbillies, we're left with this sort of generic southern/western cowboy/hick drawl, and a handful of easily written, easily read print artifacts that immediately identify the speaker as a backwoods rube. In fact, back in the 1970s I had the misfortune to read a contemporary translation of Lysistrata in which the translator gave all the Spartans southern accents, to convey the Athenian point of view that the Spartans were all hot-headed, poorly educated, country bumpkins.
Is it fair? No. But what are you going to do about it?
Y'all come on back with yore comments now, ya hear?