Because he finds it tiresome and not fun to prepare and play the World Championship. It is months of work. And he think quicker time controls are more fun OR chess960 with classical timecontrol. Basically he don't want to fight lines wich his opponent has spent months with a superengine to prepare and draw him. He want to have fun playing chess.
Oh, to have Magnus' problems
Actually no, we all know Magnus Carlsen is the best current player. Because he has the highest Fide ELO. Its obvious that if he would still participate in the competition, he would most likely still be world champion. Except if he would have had really a lot of bad luck.
About his motives I can only speculate.