Another factor is that everyone likes the music that he/she isn't used to. I mean if you listen to music of the same genre or the same influence a lot you will always get bored of it. And you look for something new. In this interesting case the old songs are the new ones, they sound different. And there are so many variations in old songs that you will less likely get bored of them.
I agree that synthesizers are generally less interesting than acoustic instruments. I defend this by saying oscillators always use the same algorithms for different notes, but in acoustic instruments every note has its own quality. Which is why violin players are so picky about their strings and fingering and timbre.
I feel like popular music today takes the music less seriously. Why make good music when a song with a pop culture reference gets more attention?
I'm thinking mainly of gagnam style, that facebook song and the big bass song. Is music moving from an artform back to a party gimmick?