I believe that it is because it's simply more enjoyable to listen to. The songs are still a relateable as they were in the 40's-60's, All the instruments are real, With real musicians to give them more power, And Sinatra's singing was so clear one never needs the lyrics to understand what he's saying. Combine a great singer with great bands, And you've got some of the best music of all time.
There is techno 80s 90s and dubstep, The guitar has evolved and so has the electric piano and synthetic music. And thanks to the miracle of auto tune you can reach any note.
The only reason it might sound good is because of nostalgia but then it wears off.
That and the new rap music every listen to. That rap music fad will pass over and other types of music will return.