Because Fallout MUST have a 50s theme song to clash jarringly with the grimness of the Fallout Universe, and yet have a subtext that makes sense within the story. Even if we can't put in-game due to copyrightz, something to put on my AoP playlist is just cool.
Fallout had "Maybe" from the Ink Spots.
Fallout 2 had "A Kiss to build a dream on" by Louis Armstrong
Fallout New Vegas had "Blue Moon", which is a cover by Frank Sinatra of the original 30s song made by Richard Rodgers.