Well the Fallout universe divides around WWII-ish, after which I would assume culture went in a different direction, and things developed differently, so they arrived in what we would know as 50's culture in the 2070's.
Another possible reason is that culture kind of stalled in the 50's while scientific advancements were made, such as Laser and plasma rifles, robots (Some of which have the ability to hover a foot-ish in the air), fully functioning AI systems that talk, power armor, shelters from nuclear blasts, a gigantic robot with laser eyes [http://melancholy-protection.net/lolwut/gaby_libertyprime3.jpg] and various other things which we take for granted in the post-nuclear-holocaust world the games take place in