This is because, Bruce Lee came of an age where martial arts was a new discovery to most of the western worlds. However, the fascination has now disappeared with the introduction of martial arts schools and countless martial art films. As the intrigue and mystique that was caused by Bruce, has now dissipated into what has become his legacy, never to be repeated.
I agree. with the Jackel. I think Jackie Chan has not only come close but surpassed Bruce in popularity. Don't get me wrong Bruce Lee was a superb exponent of his arts and had bucket loads of charisma. But his career was short-lived (sadly) and it is his legacy and people's memories of him and his films that has assisted popularity. There were other big martial arts stars in Asia before and after him. But the west's love of kung fu films waned much in the same way as our fondness of Australia did when we forgot about Crocodile Dundee II.
I think it also has a lot to do with his untimely death whilst still the public eye. The same can be said for Bob Marley, John Lennon, Marilyn Monroe, James Dean...