There was a 10 year or so build up prior to Nevermind where punk (and it's variants) were not being played on the radio (obviously, which was still relevant then). You didn't hear Black Flag, Minor Threat, Hüsker Dü et al. on the radio except perhaps a specialty show or on college radio. That pressure was building every year and so was the fan base. People were tired of hearing The Who all the time..."corporate rock".

MTV's 120 Minutes was essential stuff back in the day.

I grew up in Seattle - friends were telling me for years before Nevermind about Nirvana. They were building a reputation slowly across the US. Subpop was helping, they had a national network, but not fast enough. Once Nirvana had a major label marketing for them - things went crazy.

Nirvana was that magic mix of punk/hard rock and pop that broke the dam - and the audience was ready to go.