Why is classic rock such a huge marketing phenomenon? Because it's safe as milk. Because the guys who took over FM radio programming in the late 70s were former 60s popular artists (look up John B. Sebastion). It's because there are huge congloma-stations like Clear Channel, etc. There are a lot of reasons "classic rock" is "popular".

But the last reason is because the music is any better than lots of stuff that came after. I don't think that's true at all.