Why set something in the 50s...
...and then ignore how they spoke/behaved?
Now put that gun down Heidi! You’re not solving anything by massacring people!
Are they ignoring they way they spoke in the 50s or is that the way people spoke on film and screens?
Dominic West affects that pronounced tone on the TV show scenes.
I recently saw a programme about Ronnie Scott containing an interview from as late as the 1970s with two young MPs who regular visitors to the jazz club. They both talked in that slow clipped received tones even in the late 70s. The MPs were Ken Clarke and John Prescott.
I'm not sure about the way people spoke at the time (I was only 5 at then) but I can say they that time travel has not been ignored. If you watch reasonably closely you can see telephones that didn't become available until the mid 60's.
Bel's PA has a Post Office LST4 (Loud Speaker Telephone 4)on her desk. It's the large beige/brown box with the handset on the back. This was available only from 1966.
Various other later phones keep popping up, usually identifiable by the plastic coated coiled cables.