Well, IF there is a real phenomenon of it, which I don’t think there is. I remember that as each new genre of music took hold as I grew through my teens and onward, that the immediately previous genre was declared “out” by the next micro-sub-generation, and it left the airwaves and was replaced.
But that was mostly because the BUSINESS MODEL that had been developed during the 1940’s and 1950’s, still controlled both the music promoters and producers, AND the radio stations they all used as tools to retain control and profitability. Throughout the entire time that artists were proclaiming freedom and rebellion, the same business people stayed in control of the actual main businesses supporting everything. They even turned rebellion into a new formalized fad, and started making money off of THAT as well.
Right now, we’re going through an expected cycle of 70’s era stuff being brought back. I think it might have to do less with the staying power of the material, than with the age and positions of power of the people who grew up with it as kids. But I don’t really know.
For myself, I went through a long period where I couldn’t STAND the 70’s stuff, even though I loved it at the time. Some of the stuff that was brought back recently (such as the soundtrack of Guardians of the Galaxy) was stuff I actually did hate AT the time it came out, but now I find I really like it.
Maybe it’s that there was a LOT MORE well-produced recordings of more kinds of stuff than ever before, in the 70’s. It WAS an era where more people had more spare cash to spend on entertainment than any previous generation, after all. And it coincided with advances in recording and reproductive technology, that made it possible for more people to enjoy it.
Something else to consider: seventies music ITSELF, wasn’t a single “sound.” It was, from beginning to end, produced by people who grew up on and built on the music that came before. There were throwback-type songs made back then, as there are now. So some of the reason why it seems to have “staying power,” is that it was built from other music that had "staying power.
Just rattling on at this point. There are lots of reasons why people like stuff from back then. Maybe it would be easier to try to figure out why we are NOT still hearing the stuff that you see as having dropped out of sight. Or hearing, rather.