The phenomenon of Mid-Atlantic English appears to have been a class marker rather than merely people being embarrassed about having a regional accent.

From https://en.wikipedia.org/wiki/Mid-Atlantic_accent :

It is not a native or regional accent; rather, 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". The accent was embraced in private independent preparatory schools, especially by members of the American Northeastern upper class, as well as in schools for film and stage acting