The reason is (according to Carlsen) of course that FIDE is driven by a strict adherence of «rules» which are defined by a small set of people in power. Whenever something happens they always say «oh, but these are the rules», but the process for changing the rules is very one-sided and power driven. This was the straw that broke the camel’s back. Yet another example of a silly unnecessary rule.
[1]: In an earlier WC he got stuck in traffic and arrived in ski clothing, but changed after the first round.
This every government or organization that has ever existed. Every human group from beginning of time. Left or Right, up or down.