Leon hit it right on the head.
You CAN find something to like in modern and popular music. I like Linkin Park, their hooks sound great and their lyrics are nice and aggressive. As long as they keep making music I enjoy I'll keep buying their records. Are they as tight and timeless as a band like Metallica? No they are not and I don't put them in the same catagory at all.
Another thing is that most bands for the young are too old for them to get into and because of that they just take what is current and sounds fresh to them. Throw peer pressure into the mix and there you go.
Someone like Christina gets popular because she does have a good voice, good looks, and I hear her stage show is excellent. Sure the songs are derivative but most of the business of music is visual and not audible. KISS could be guilty of the same thing as their music is very safe and hardly top rock or metal quality yet they put on a look of danger and wow the audience with a matinee style gore show.
Elvis is a great musician but I only ever knew 1 person all the way through my school years who admitted that he liked and listened to Elvis.