Since then, hypotheses and answers have flooded in. Here is a first installment. More to come.
From a reader in Canada, the accent died because it never really lived:
The reason it disappeared so quickly is that it wasn't a "real" accent. It didn't come from a particular place and it wasn't used by regular folk. It had to be invented and taught. So as soon as acting schools and elocution teachers stopped teaching it, it stopped being used. It would only take one generation.
I think the disappearance of this accent is a good sign -- a sign that North Americans have embraced their regionalisms and the richness and variety of their own true accents.
A similar thing has occurred in Canadian French. Something called "International French" (a completely artificial accent) was once the norm on radio and television. But now you only hear it in classical theatre.
It was fake, but it was based on something real:
I've been fascinated by this accent for years, and have known for a while that it was taught to actors and announcers in the 30s and 40s. But I do believe that it was based, at least in part, on a certain now-vanished Northeastern upper class diction.
I have lived in and around Hartford since the mid-70s and in the 90s attended an Episcopal church that drew a lot of its membership from the same old Hartford Yankee WASP milieu that produced the Hepburn family. I got to know a couple of smart and delightful older ladies who had actually grown up with "Katty," and I am telling you that they had That Accent. A bit softened around the edges, probably because it was so long out of style, but still fully recognizable.
More on fake and real languages:
The rapid disappearance of an artificial actors' accent is no surprise -- it was never part of the living language. What happened to it was Humphrey Bogart, Marlon Brando and a more naturalistic style of acting. If you want a more surprising example from living language, listen to samples of the Queen of England back in the 50s and now. She's definitely much less pinched and posh than she used to be.
P.S. Katherine Hepburn's accent was more of a genuine east coast upper-class accent - learned at home, not in acting school. Thus it would sound closer to modern speech.
The Yankees did it:
I suppose that elocution finally passed from the everyday once Bob Sheppard ceased his work as PA announcer at Yankee Stadium. Why is an interesting question. Was it a shift toward a more "personable" voice? Was it the replacement of newsreels with the small screen and the radio with images? I wish I could remember more about the tone of the narrative voices at the NY World's Fair, but I wonder how much was brought on by a shift in the culture, how much by the medium.
It's like Shar-peis:
Didn't Cary Grant have some of this flavor? How about Gregory Peck? Were these the last living examples?
Of course an accent that exists only by dint of deliberate effort can vanish much more quickly than a "natural" one. How quickly would shar-peis go extinct if people did not take the trouble to breed them? I could imagine this accent falling out of favor during the 60's, when it would become identified with the unfashionable "establishment," as the Cary Grants and Gregory Pecks of the movies were yielding ground to the Jack Nicholsons and Robert Redfords.
Elvis did it:
I read something not long ago, I wish I could remember where, about how the rise of Elvis Presley was the beginning of the "Southernification" of American culture. Watch almost any movie from the 40's that took place in present time and the accents are of the same vein as the Transatlantic voice. I've always thought of it as a Philadelphia voice. Kate Hepburn to Jimmy Stewart in The Philadelphia Story "where are you from? South Bend? Sounds like daancing." If you play Walter Cronkite in your head you can hear the same cadence as can be heard on this video, he's obviously less arch and his lower voice range eliminates some of the high edge but, his voicing is from the same ilk.
Elvis opened the door to a whole library of American culture that had not had a place in radio, movies or this new fangled thing called television. As more and more Southern and Western raised talent gained success and a following in the national culture, less formal speech became more acceptable to the ear. And, since there were many more people like Elvis in these great United States than there were people who spoke like Gayne Whitman the more casual, less affected voice quickly became an accepted norm.
It was because of method acting:
I bet that with the emergence of method acting as the de-facto acting technique in film and theatre around the early-mid 50's, the emotive accent of classical instruction (the transatlantic accent) was replaced with more lifelike accents as a matter of craft.
Just a theory!
It was the loss of poise and manners:
Americans stopped talking "this way" when poise and manners slid out of favor in the public media. This coincided with the rise of cool and "irony." If you made a graph of the decline and rise, I suspect the two lines -- the decline of fine speech and the rise of cool -- would mirror one another. I bet the point where the two lines cross would mark the demise of stockings, men's hats and neckties. Since then we have all dressed (and behaved) more and more like children. (I put "irony" in quotes because what passes for irony these days often has a large dollop of smugness as part of it, and true irony is not smug.) Wearing a ballcap in church is not ironic. It is rude.
So the next question is: what started the rise of cool and faux-irony? I think it could probably be pegged to the arrival in our land of overabundance. When Americans stopped talking "this way," they stopped worrying about being presentable so they could get or keep their jobs. They no longer felt they had to be able to impress others with their maturity and knowledge and respectability. It is hard to say which came first, the decline of manners or the slide in education and public intelligence. But it seems certain to me that one leads to the other and the interaction of these two things fuels the downward spiral of our culture.
Teddy Kennedy illustrated the change:
Even though the announcer's accent was synthetic from the get-go, it's still true that national accents have changed radically since the 1940s. You can easily see that if you stumble across a rerun of a show like "You Bet Your Life". Those common accents are now wondrous strange.
But to my mind, the most interesting example is from a single generation in a single family. Listen to Ted Kennedy and compare him to his older brothers. I lived in Boston from the 1970s until just a few years ago, and I never heard Jack's accent on anyone younger than 70. It has vanished other than as an affect. But Ted's accent changed with the times, and was pretty much lingua franca at the end of his life.
'I'm glad it's gone':
The first time I ever heard ['that' accent] was when I caught William F Buckley on TV while channel surfing.
I had no idea who he was or why he was talking like that. It sounded vaguely foreign and strange. But, it wasn't foreign in a way that I had ever encountered before. Remember, I grew up in an immigrant family in SF and, in a typical day, I would hear Californian English, Taiwanese, Mandarin, Hakka, Japanese plus German and Latin in school.)
Then I read about how Hollywood imported British actors to lend some "class" to the movies. Then all sorts of broadcasters took on that affectation.
When I hear the accent, I think of it as a pale version of the confederate flag. It says 'WASPS only.'
I am happy not to hear that accent any more.
More in the queue. Thanks for these varied and intelligent responses.