Fischer did not defend his title because the Chess Federation refused to accept Bobby's score keeping system during tournament play. Fischer wanted to reward those who showed aggressive play. He could not stand the scoring system where a player could get one game ahead and play for draws the remaining games and declare victory.
Hopefully this answers your question if you had one as to why Fischer dissapeared from the chess community. Plus the US was after him for income tax evasion and he had to lay low everywhere he went.
Bobby was indeed the best chess player ever, considering that he played in an era where the whole world was competing. I still appreciate Morphy, Lasker, Alekhine (sp?), Steinitz, and many others who contributed so much to chess through their creativity.