Slate has an excellent article tracing the root causes of the recent demises of Vibe, Blender and other music magazines.
Their three reasons for the decline of the music mag:
1. There are fewer superstars, and the same musicians show up on every magazine cover.
2. Music mags have less to offer music lovers, and music lovers need them less than ever anyway.
3. Music magazines were an early version of social networking. But now there's this thing called "social networking."