Nadal's rise to fame is basically because he represents the anti-Federer. Yin and yang. Not any better or worse, just opposite. Federer plays elegant and offensive attacking tennis while Nadal plays brutal and gritty counter-punching. Federer is the most naturally talented player to ever play while Nadal is the hardest worker. Federer exudes class and skill and Nadal brutality and youth. Anyone who dislikes one is bound to like the other.
Nadal definitely has character on his own but his traits are magnified by comparing him to Federer. Together they have achieved higher popularity than they ever would have separately. For Djokovic and Murray to achieve this kind of popularity they will have to take their rivalry to the next level.
LOL, I should have read the thread to make sure someone had not already posted the exact same thing I did.