Why does grundge and alternative rock influence more teenagers than hair and classic rock?

glam metal and classic rock influenced me growing up

The generation that were teenagers when that stuff was at its apex are now the parents of those teenagers and they're playing it in their homes and their kids are being exposed to it. Hair will become more and more obsolete. Classic rock has a certain relevance lasting power and probably still holds a sway over a certain percentage of kids, probably will for the foreseeable future, but grunge/alt rock still exists in those households because the people at the heads of many of them lived those bands in their formative years and thus the kids are hearing it, and some percentage is liking what they hear.

I disagree with your premise.