It's also to do with a film industry that looked to English theatre for its credibility - and at the time English theatre taught actors to use Received Pronunciation (AKA "BBC Englinsh", "The King's/Queen's English") and emulate upper class, southern home counties voices, so all of our actors (and consequently all of yours) sounded like well bred southern englishmen - basically the way I do (not that you've spoken to me). Think David Niven or Noel Coward, then tone it down a notch, and you get Dirk Bogarde.
In the 60's America found the Beatles and the Stones, and realised that England has more accents than you do in 50 states, in a country smaller than most of your individual states.
In the 70's US cinema found its voice, and method acting as mentioned above.
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Overhead, the Albatross hangs motionless upon the air,
And deep beneath the rolling waves,
In labyrinths of Coral Caves,
The Echo of a distant time
Comes willowing across the sand;
And everthing is Green and Submarine
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