Hendrix took one genre of music, the Blues, and reinvisioned it for a new generation, in the prosess he created a new genre of music. This music was vastly different to what was in popular music at the time. It sounded nothing like the Beatles or the Beach Boys or the Rolling Stones And although there were bands that were doing similar things to Hendrix at the time, bands such as Cream and The Who, Hendrix took it to another level and inspired just about every great guitarist since. He changed how Rock was played he ripped apart conventional blues with a creativity and imagination that has never been seen since.
Cobain was a great song writer but his genra, punk, (grunge is a marketing term invented to sell fashion) had been well established in the seventies. his music in particular was not that diferant from the pixies or even the sex pistols what made cobain and nirvana rise above the other great punk bands of his day was the passion and emotion of his performance. This is why when you hear a nirvana cover it never quite sounds right. becauce cobain and his music are one and the same however hendrix music can be performed by other musicans and still sound good. His music stands on its own