In fact it was notable for not being anyone's native accent! Although it was supposed to capture some of the perceived "classiness" of some British accents.
> According to voice and drama professor Dudley Knight, "its earliest advocates bragged that its chief quality was that no Americans actually spoke it unless educated to do so".
You could think of it as an effort to unify the way people speak English in the US. Not a bad idea I think, if it works. Less "otherness". I read somewhere that many people with Brooklyn accents were taking lessons to get rid of their accent.
I had som much fun when I met this girl who spoke like the girl in "My Cousin Vinny". It was so "authentic". The cool thing about US is that there are so many immigrants who speak English but they speak it in their own accent. But I think it is only natural that eventually they will all speak more like each other.
Certainly there were people who spoke similarly to movies and radio, like my grandmother. However, the full truth is rather sad: Anglo-American speech at the time comprised many beautiful and very diverse accents. As media became ubiquitous, these began to die off and most seem extinct today.
But I just assumed that's the way people spoke in older times, just like they dressed differently.