If you're like many movie goers, you always seem to wonder as to why Hollywood Stars or Broadcasters of movies, tv broadcasts, radio broadcasts, tv shows, etc. all have that old-time sound to them. It's very intriguing since the English language today seems so labeled by either American or British variation. In a video created by Brainstuff, he explains the reasons behind it all.
The way of talking was called the transatlantic or mid-Atlantic accent which was a mix of American and British pronunciation. Hollywood loved it because it had no direct location as to where it derived from making it seem more unique and iconic. T's were accented and R's were dropped giving it a unique sound.
Realistically, the reason why it was around in the first place was because it was an acquired accent. It was literally taught in schools, it wasn't a typical attained geographical accent. After World War II, teachers stopped teaching the accent so it faded out to the geographic English accents that we know today. It truly is iconic to the 1930s and early 1940s. Neat!