American Accent from 30s-40s?

Why did the American accent change so much? It's not neccesarily a proper diction or grammar thing, just the flow of the words. An example of what I'm talking about is the actress who played Liesl from The Sound of Music. I know that the movie is from the 60s, but the accent is what I'm talking about. I absolutely love how she speaks and sings, you don't really hear people who sound like that anymore.