That’s not the only accent gone. Listen to any news broadcast from the 60s. Then to one from the 30s. Then a current one. All 3 are very different.
While accents do shift, part of what you are hearing may be due to the simple fact that audiences have gotten geographically larger, so that regional accents - which would be noticed by segments of the audience - gradually came to be seen as a drawback.
Further, there is another separate but similar phenomenon: the style of speaking. Radio news readers (if that's the correct term) in the 1930s had a nasal quality; perhaps it came through more clearly over the crackly ether. In the 1940s and 50s, a strident delivery and a youthful, all-American tone of voice were prefered. In the 1960s and 70s, after t.v. acquired widespread popularity, deeper-voiced, almost grandfatherly newscasters predominated. These are merely fads.
Regards,
Listen to Looney Tunes cartoons. All the characters sound like they are from Brooklyn.
I can say that I will not miss the accent. Used to work with a bunch of people from NY. "Hey, let me ax you sumpin. How would you like to eat a sqwirrel sangwich?"
I saw the Miss Americas introduce themselves in a clip from the 30s and each and every one of them had a strong and distinct accent from the state they were from. Now they all sound the same but for a slight accent from one or two from the south.