When people were first piloting the telephone and radio, and later the television, broadcasting companies started trying to standardize the American accent so that one American dialect was deemed 'normal' while others were considered rougher and less refined.

Note, for example, how similar Hollywood stars sound to each other. In movies they tend to have the same range of 'default' accent, and don't deviate from it unless they're playing specific characters. Same with news broadcasters, etc...

It's also the default accent for the telephone operator messages, which incidentally have remained essentially unchanged for decades.

And what was the place that they decided would be the "default" American accent? Cleveland, Ohio.

It's not a mistake that you perceive suburban Ohio as mainstream. It was intended to be.