Great idea for a thread, William!
A lot of this is of course down to commercial popularity, ie what the radio plays, and how much money the genre brings in, in terms of concerts, promotions, merchandising even (not too many people would wear an "I love jazz" t-shirt) but another factor is this sort of desire for the younger generation to always despise their parents' music. Many kids would put down say jazz or big band in the 50/60s because it was "old people's music", although in all likelihood they never even listened to it properly. Same thing with punk vs prog and pop vs rock. There's an almost unconscious need to put down music seen as older, to be seen as "hip" or "with it" or "cool" or whatever the prevalent phrase is at the time, by latching on to the latest trends and being with the in-crowd.
So older music generally gets short shrift from the younger generation, and sadly, mostly these are the ones who buy the records that make up the charts, and therefore determine what is deemed popular and what we will hear on the basic radio.
Then of course you get revivals, when some music is seen as "retro" and is now cool. That 20/30s style was popular again for a while, mostly due to "Boardwalk Empire" and particularly in Britain, on the back of their (failed) Eurovision entry. Sometimes it comes back around, such as with prog in the 80s, to sort of fade out again, and sometimes it metamorphoses into something almost completely different, like "New" Country. Then you have genres crossing over, so that the best (or worst) of both worlds is achieved.
One thing is certain though: as you say, in another however many years, the current trend which is seen as cool will be villified by the kids as they groove to their new favourite genre, whatever it may be. The world turns, and music changes, but one thing remains the same: our belief that the music our parents listened to was crap, and our, in general, refusal to give it a fair trial and just dismiss it out of hand.