Hmmm, Most influential martial artist of the 20th Century. Well I agree that Jigoro Kano's philosophies and dedication did much to further the art of Judo. At the same time though, I firmly believe that Bruce Lee did just as much to further the interest in all martial arts, not just Jeet Kun Do. Bruce Lee was a deeply philosophical and educated person who went against the traditional martial arts thinking of the time to create an art that blended the best of many arts (Boxing, Kung Fu, Fencing, and others). Bruce Lee's books are probably among the most widely read books among martial artists; Tao of Jeet Kun Do is popular and has basic applications for all arts.
I believe the reason that Jeet Kun Do is not as widespread as Karate or Tae Kwon Do is because there is only one Jeet Kun Do, unlike the upteen different styles of karate or Kung Fu. Not many individuals trained directly with Bruce Lee (Dan Inosanto is probably the most knowledgeable among them).
Bruce Lee's unfortunate passing at an early age also lends to Jeet Kun Do not being as widespread. Mr. Lee died during his prime, and together with the passing of his son Brandon much of the knowledge of Jeet Kun Do passed as well.
I admire Bruce Lee not for his movies, but for the way he lived his life. Bruce Lee was dedicated to training, not afraid to change classical thinking, and committed to furthering the martial arts.
I'll get off my soap-box now. Have a good day!