I heard from a friend of mine that Magnus Carlsen did not play the Candidate Matches in 2011. Why did he do that?
Magnus Carlsen drops out of World Championship cycle (contemporaneous chessbase.com article)
Largely he didn't like the format (knockout matches rather than round-robin tournament).
Carlsen wrote a letter to FIDE explaining why he skipped the Candidates Matches, quoted in part below:
After careful consideration I’ve reached the conclusion that the ongoing 2008–2012 cycle does not represent a system, sufficiently modern and fair, to provide the motivation I need to go through a lengthy process of preparations and matches and to perform at my best.
Reigning champion privileges, the long (five year) span of the cycle, changes made during the cycle resulting in a new format (Candidates) that no World Champion has had to go through since Kasparov, puzzling ranking criteria as well as the shallow ceaseless match-after-match concept are all less than satisfactory in my opinion.
In my opinion privileges should in general be abolished and a future World Championship model should be based on a fair fight between the best players in the World, on equal terms. This should apply also to the winner of the previous World Championship, and especially so when there are several players at approximately the same level in the world elite.
One possibility for future cycles would be to stage an 8-10 player World Championship tournament similar to the 2005 and 2007 events.