As for the real reason why they're bitter, it's because despite Juan's unfortunate wrist injury/surgery, it took their man 3 years to finally do what Juan had done and win a Slam himself - and even then he did it in circumstances we know, a far cry from Juan's blasting through the two most dominant players of the XXI century. Deep down, they know the truth hence the bitterness
Even though Juan denied Federer a Slam title and Nole an Olympic medal and gave Nadal a good hiding in USO semis, you don't see any particular bitterness from the fanbases of Federer, Nadal or Djokovic, do you? Figures.
There's plenty of shite the opposite way though, I must have seen at least close to double figures in Delpo fans bemoaning the fact 'tennis died' at the US Open, when I translate that into the fact they don't get to gloat endlessly about the whole 1 > 0 thing anymore, which is good for both fanbases really.
Coming from a fan of a GS champ to another