Some of the "Glam rock" acts tried to become a part of the changing climate of rock music, they went so far as to change just how they sounded. The "glam rockers" undoubtedly carved out their own sounds for years, but once Nirvana showed up on the stage, everyone wanted to sound like Nirvana. It's like KISS doing disco back in the days. When disco died, they carved out their own sound.