It's simple Peggy, good music never dies and the newer stuff has been so generically processed in the last 30 years that the chord progressions mirror each other almost to a fault. It's not called industry standard for nothing anymore. The boy band boom of the 90s only cemented the clone idealism of that music was no longer important to sell the product anymore.
Plus, "old music" hasnt totally disappeared from the market with Sirius and cable music services still providing a vast variety of music genres to listen to. Younger ears have probably been exposed to their mom&pops record or CD collection or dare say grandpa and grandmas since 50s60s.
This music still holds it's musical roots in Elvis and the Beatles whose names still pop up in younger artists as far as influencing them from music to image. Although sadly, those influences rarely show up in their music. (sigh)
Let's face it, the older stuff is more creative in chord structures and style then the generic predictable 3-4 chord bebop stuff being passed off as popular music the last 25 -30 years. I mean, there's a reason why Dylan and the Boss's catalogues sold for millions because that music simply isnt going away.
Im sure all of the Beatles offspring will never have to work for generations on end granted they have a trust set up for them. Again, this trend of "old music" continuing to sell is not by accident. In fact the Zombies Odyssy and Oracle album has sold more copies in the last couple decades than it did when it was released.
Again, what is old is new again to "new" ears. At any rate, this would be the time for another retro band to do the music that made the 60s&70s great as in how Jet sprung onto the seen with song "Wanna Be My Girl" that was featured on an Apple IPOD commercial in the early 2000s.