Loud-quiet-loud was the signature sounds of Nirvana - borrowed from Frank Black who pioneered it with the Pixies. Kurt was a way better singer and better looking than Frank Black. Along with good songs, that was a recipe for success.
I am a big Nirvana fan. But a bigger Pixies fan. A chunk of Pixies sound/energy was inspired from the guitar middle/outro in B52's "Rock Lobster".