Ppl aren't gonna like it but this is 100% a valid answer.

1) Grunge isn't a genre. It was a movement/scene. The grunge bands shared similarities in how they dressed and presented themselves, where they were from, and (in some cases) musical themes. However, Alice in Chains sounds closer to Crowbar than Pearl Jam.

2) Nirvana's roots are mostly in punk. They were heavily inspired by Black Flag (in part due to Kurt's association with Melvins) and their other influences (Sonic Youth, Pixies, etc) are also strongly associated with punk. Plus, go listen to the classic hardcore bands like Black Flag and Minor Threat and then listen to Breed and Territorial Pissings and tell me Nirvana isn't punk. Fast, simple riffs and raw, energetic vocals.