I also had the same thoughts about the series before Fallout 3. That things did advance culturally but differently then our own. The 1950s mindset never went away but things still changed.
I would like to think there was a Vietnam war, only it played out differently and never escalated to the point it got to in our own timeline. The people never lost faith in the Government. "The Government knows best" mindset stayed and the counter culture revolution never took off.
I also speculated that as the world got closer and closer to falling apart the American government got more authoritarian and things reverted more towards the 1950s style that was always there but became more pronounced at the end.
Even in Fallout 3 it shows that things did change dispite Bethesda going way over board with the 1950s thing. There seems to have been a civil rights movement at some point.