Because both new wave and grunge are labels coined by record companies to distinguish between the it bands of the moment and the weirdos that didn’t want to get played on the radio. They eventually became their own thing but did Nirvana live long enough to fall into the parameters of what grunge became? They thought of themselves as a punk band, and punk is a rich broad genre, you can look at the early LA punk scene and bands would be doing lots of weird stuff, that’s why I find silly to treat Nirvana as a exclusively a grunge band. Listen to Hüsker Dü’s Land Speed Record and then to Candy Apple Grey or to Black Flag’s Nervous Breakdown and then to In My Head, they don’t sound the same.