Here is one possibility -  music is a classic meme, at least to memeticists. They define memes as a sort of replicating unit of transition, a replicator that behaves as a cultural brother to genes. As genes they are not resistant to changes, they evolve, they get extinct, some change more often, others are more stabile, even dormant. I find this meme theory very interesting but I am not sure I can embrace it fully.

It probably results from the relationship between teenagers and parents. A teenager who is trying to appear "cool" in front of their friends will tend to enjoy music that their parents don't like to satisfy their rebelious nature. After all, the music your parents are interested in is considered very "uncool". As a result of this, the music produced will change.

Another theory is that musicians, in order to stand out in the music business will try to create a new sound, because if they don't, they will go un-noticed. As a result of this the music constantly changes.