These are actually two great points. By the time The Strokes and The White Stripes came around, critics went nuts for them (except for the Vines, who I remember got a lukewarm reception) and the mere idea of Nu Metal seemed like a joke. Also,pretty much everything you said about 9/11 putting a big nail in it's coffin. Granted, modern Rock radio hasn't changed that much. There might not be as much Nu Metal on the airwaves today, but instead it's Nickleback and other Post-Grunge and butt rock bands. Interestingly enough, even that radio format is slowly dying. Pop music is now taken more seriously than it was ten to fifteen years ago, and Hip-Hop remains the most popular form of music. When you look at it like that, a bunch of shitty bands with frontmen that have really annoying, yarl-ey vocals is something that not a lot of people want to associate with.
Another thing to factor in is the rise of mainstream Emo and Mallcore. Sometime in the early to middle (and in some cases late) 2000's, bands like Dasboard Confessional, My Chemical Romance and Hawthorne Heights became popular. For kids who had tired of Nu Metal outside of maybe Linkin Park, these kinds of bands were the soundtrack for the era of Livejournal (remember that?) and Myspace. To them, it was probably "You meatheads and bros can have shit like Drowning Pool and Limp Bizkit, we'll stick with this." I'm not saying it was good music (I'd rather not go back to an era in which Good Charlotte is a thing) but it probably helped play a part in Nu Metal's decline in popularity.
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