1) He was the First one who took guitarplaying to the next level and his technical ability and showmanship is better. (ex. playing by teeth, burning the guitar and playing the guitar upside down).
2) He put guitar in the awareness of everyone, and proved that it can be used not just as a back-up instrument for a band/ vocal accompaniment but you can be a star on your own by just playing guitar as a soloist. Which paved the way for other great soloist...Vai, Yng, Satch, EVH, etc.
3) He was a household name back in the 60's up to now. While its undeniable that Kurt Cobain does have a lot of followers and fans and he has established his mark as the "Grunge King", His scope is limited by time, remember Nirvana became famous in the early 90's while Hendrix has already influenced players spanning 3 decades.....so Hendrix is definitely more revolutionary.
Case Closed ;)
Guitarplaying is 1 Percent Inspiration, 99 Percent Perspiration... :)