Short explanation: the music industry has discovered that capturing the teenage market pays dividends and teenagers are emotional. Unrelated anecdote with a different explanation: I had a fascinating conversation with my parents recently. They are both children of the 60s and grew up in San Francisco. We often share music just to talk about it. I was playing some heavier (dubsteppy-metal) music to them, and wanted to…
The rebellion was very much against the machine, the establishment, the norms,etc.
It's very interesting that nowadays rebellious trends tend to feed the machine. You were spot on on your first line. Whatever captures any market and pays dividends is echoed right back, amplified by the machine (which takes the dividend). There is still great music being made, but it's harder to find (if not impossible) on mainstream channels.