Grunge, or a different type of rock music separate from hair metal, which was big at the time, was something that was being pushed for at least a year or 2 before Smells Like Teen Spirit and Nevermind in 1991.
Soundgarden (another Seattle grunge band) had popularity before almost anyone had heard of Nirvana, Louder than Love, 1990, was on A&M, I had the album, it was on the album chart, Mudhoney was somewhat famous before Nirvana as well. It's not all that entirely clear that (((they))) picked Cobain out. May very well be that Teen Spirit was just a little bit better than Touch Me I'm Sick, in the minds of some people.
That year, or that era, there were a ton of bands getting mainstream famous, bands that were around a bit. Metallica was underground famous throughout the 80s, and then, Enter Sandman got them a whole bunch of new popularity. RHCP got superfamous after being less famous. There was just a whole lot getting really big around the same time.