They aren't major chords, they're mostly power chords, which are neither major nor minor (no third and the third provides the major/minor tonality). They often function as minor chords because of the melody or other parts, or just because of how the progression fits together. They aren't unique or new with Cobain, he was part of a long history of punk and rock and roll.

Cobain was a good songwriter in the rock and roll tradition. He was not particularly innovative or doing something technically unheard of, and he wouldn't have claimed to be. He wanted to be a good songwriter, and he succeeded. That's it, don't make up bullshit about it.