Because it takes a while for new ideas to get absorbed into the culture, it's inevitable to have cycles. For instance, a lot of what Jim Hendrix did took a long time to become part of the language. Finally, there was a reaction against it. There are a couple of things. A lot of it has to do with money.
Musicians are playing it safe because the money is uncertain, so they try for the big stakes.
Whereas now, it means moving into the neighborhood you've always wanted. Because of rock's popularity, it's possible to make more money than ever before. The other thing is that like a lot of cultures, we have our pop music and we have our art music. The most popular stuff follows certain formats and tends to be very simple and direct, but sometimes you can't get by on just that. Sometimes you have to have a lot of depth in music, and depth often means complexity, although some music is simple and deep.