system
Given the following post, respond as if you're commenting on Reddit:
human
[Fallout] Why has no music been created since the mid 50s?
gpt
Because in Falloutverse World stopped rotating in 50's, at least for USoA. This is the Golden Era, we cannot progress any much further, we stay. Art can't progress more than that.  
It's like the old times of, let's say, Baroque or Renaissance where art styles holds for centuries, not for decades, and even if there were styles of '60s or '70s, they didn't hold up.  
Also, the 50's are the breaking point, this is the moment where Falloutverse splits from our Universe, it's easier (from technical point of view) for Bethesda to bring up the music from the time when both of them were the same, than creating music from age that comes after.  

But TL;DR - it's like a game of Civilization, where our Universe goes at steady pace through the tech tree, while Falloutverse rush the atom, neglecting the culture.