My generation, the last of generation X, probably wanted to express existential angst with loud music such as altern rock and metal – music centered around distorted guitars. Metal took more hopeful '50s and '60s guitar music and turned it into darkness and despair, just like the hippie philosophy and in a way the general world view did. The millennial generation – my younger brother – clearly follows a different pat…
I would also argue metal was never mainstream. The mainstream was always further takes on pop, which to an increasing degree draws from all the others innovating, if in a moderately expressed way. Listen to mainstream popular music now and you can often find some of the intensity extreme metal explored.
Come on, it is/was, at least in Europe. Plenty of famous metal bands in the late 90's early 2000. Even death metal had a few commercial successes.