How and why do accents change through the years?

Oh how I love the voices and accents of old-time radio or movies from Hollywood.

The american accents seem to change quite a bit in merely 50 years or so. It's personal opinion for sure but I do prefer the actor's voices in Cassablanca then current day TV news anchors. That makes me wonder what makes the accent change through the years.

You are probably hearing the Mid-Atlantic accent in older films, especially those from the 1940's and 1950's.

It was an accent that was preferred by the elite and Hollywood throughout the early half of 1900's.

Primarily fashionable in the 1930s and 1940s, the accent was embraced in private independent preparatory schools, especially by members of the Northeastern upper class, as well as in schools for film and stage acting. The accent's overall use sharply declined following the Second World War.

But it had to be taught. Rather than being natural, it is a mix of American and British English.

Instead, according to voice and drama professor Dudley Knight, it is an affected set of speech patterns whose "chief quality was that no Americans actually spoke it unless educated to do so".