One Point:

Nirvana and Korn weren't generic when they came out. They were actually remarkably different from everything else in the main stream. At the time, "Hair Metal" was the over-abused generic and "the masses" were bored with it. Then BAM! Smells Like Teen Spirit hits the radio and there's a brand new sound. Nevermind that there had been bands before Nirvana that sounded similar, but Nirvana broke into the big picture, opening the flood gates for everything "Alternative." So now there's a lot of bands that sound really similar and the newer they get, the less imaginative they sound (for the most part).

Thus the masses, tired with Alternative latched onto this new stuff spearheaded by Korn which has become known as Nu-Metal. They'll get tired of it pretty soon and something new sounding will come along and in a few months everything will sound like that...

And the cycle shall continue.