This changed through television and an intensive effort in schools to standardize spoken English, including speech therapy for people that had accents too far outside the prescribed American access. Even people learning English as another language often undergo accent remediation lessons (my wife is from Thailand and had to take accent improvement lessons as part of her TOEFL and ESL studies). Growing up the variation was extreme - there was a “middle” dialect that span the Midwest and western, but the north east, south, and various regions like the dakotas and others had very distinct dialects that were often unintelligible. This also ignores Ebonics, or urban black dialects, as well as other sub dialects.
As others point out TV and Hollywood also played a huge influencing factor here, but TV and Hollywood also existed before the convergence of dialect happened and were just as if not more important. It really came to a head in the 1980’s through 1990’s with the mandatory educational standardization of language and dialect, partly exemplified by no child left behind and related standardization efforts.
But I think culturally there are hundreds of cultures in the US, not just by region, but my demographics within a region. Black culture in Atlanta is very different than black culture in rural Georgia, is very different than Vietnamese American culture in Atlanta, etc. People from Europe are always looking at their monoculture and it’s internal variance and looking for a similar single dimensional variation. But America is culturally extraordinarily varied along many dimensions owing to its enormous communities of minorities from every other culture on earth, adapted over generations into something unique in the world to that geography plus ethnicity plus other dimensions. I’d almost say it’s so varied that it’s easy to overlook and just focus on the Generica experience presented by mass retailers and franchises.