http://en.wikipedia.org/wiki/Retro-futurism
The purpose of this style from a design perspective is to juxtapose the optimistic imagery of pop culture in 1950s America with the apocalyptic paranoia that underlined the same era as a result of the Cold War.
From an in-universe perspective, the series takes place in an alternate universe where the U.S. culturally stagnated in the 1950s.