My thought? Because the new devs and/or management either have misconceptions about the setting of the original series, or want to keep it excruciatingly simple to 'get' the concept of the FO world setting. Alas it's become 'It's the future, but all wacky and done like in the 50's or something'.
Culturally one could extrapolate that there was no Woodstock, and that the fifties aesthetic pervasively influenced every decade that followed; however it's nutty to simply assume that the only music in the world was actually recorded in the fifties. FO1 and 2 used their (SINGLE!!) fifties track as a nod to the way it had been before the war ~there was no fifties style music highlighted by the developers in Fallout's present day ~and they had ample option to simply commission music done in the 50's popular beats.
Sure, there could be original 50's tunes played in places... but the majority should have been from later artists; for instance like http://i271.photobucket.com/albums/jj125/Gizmojunk/Alternate-Earth_zps53e52bc9.jpg and http://i271.photobucket.com/albums/jj125/Gizmojunk/IggyPop-1-1_zps357123f7.jpg; or http://i271.photobucket.com/albums/jj125/Gizmojunk/M_Manson_zpsd681db6e.jpg.
**And of course they're all faked images for the fun of example... they certainly don't have to use real celebrities, but the point is that it's an alter-Earth setting where they ~and many others~ could (and probably would) exist, but have been of noticeably different countenance, appearance, and influence in that alternate reality than how they appeared in ours.