what nirvana did in and of itself was innovative because they successfully mainstreamed what a bunch of crazy underground bands had been doing for years already (mudhoney, tad, "my war" era black flag, melvins, beat happening) by sweetening it with pop hooks and arena rock style production. they were by no means the only band that did that, of course. before "nevermind" was released they were actually one of the lower rung bands in their own scene. in the end, none of their contemporaries were nearly as successful but that's how it goes.
there are multiple examples of that all throughout rock history, even. like why were metallica the anointed ones who got to headline stadiums when there were tons of other thrash acts on either coast that had basically the same sound? i mean they were barely removed from garage band status even after they had released an album. as musicianship goes their rivals in megadeth smoked them. they happened to do the right stuff at the right time which i believe to be the case with nirvana as well.