The Midwestern accent was prevalent in the media in the 1930s-1950s. Since Opie it’s also been flattened. The California radio voice rules all. Yes, it happens to be my accent, but I am sad about it. I love all the different accents of this country.
Of course when SoCal became the center of media, especially the destination for people with dreams of stardom, the midwestern accent took over SoCal. The California radio voice is the midwestern accent.
The Midwestern accent was prevalent in the media in the 1930s-1950s Interesting. Another tidbit is that the accent that was prevalent in the movies of the 1920s, 30s, and 40s was called "Mid-Atlantic." It was actual a real American accent, spoken most prominently by the upper class and those in show business.
The Mid-Atlantic accent was a combination of a New England accent and a British accent. FDR spoke with a Mid-Adlantic accent. So did Katherine Hepburn, as did countless movie and radio stars.
Here is what is sounds like:
Why Do People In Old Movies Talk Weird?