"Although the game's main theme was originally announced to be composed by Nobuo Uematsu, Hamauzu was assigned to it after Uematsu was selected as the composer for Final Fantasy XIV, making XIII the first entry in the numbered Final Fantasy series without Uematsu involved."

Basically he gave up doing a main theme in favour of being numero uno when it comes to an entire online game. I'd say it was a pretty easy call for him/SE.