Because it's not a 50s theme. In the Fallout universe, a split occurred in the time-line, and the world didn't really move on from Atomic Age ideals. While Atomic Sciences flourished, many ideas never came to surface like the integrated circuit, which is why we still see vacuum tubes.
:tops:
Its their year 2077 onward, but various cultural and scientific events did not occur, while others (that did not occur for us) did for them. It also cannot be taken
too literally, as it was already set (prior to 2077), in the "idealized made true" future of their 1950's pop-science, fears, and cultural of expectations what was to come ~and then it was blown all to hell by atomic war.