He didn't. He left because the NBA is his dream. More specifically the Bulls or the Wolves. Both midwest, both former homes, both people he likes.
Bulls are ready to win now, if that job didn't open I bet he talks to people but decides to return to go for NCAA run and then next year take the Wolves job when Flip Saunders steps back to just FO role and Wiggins is in 3rd year and Towns is 2nd year and you probably got another decent pick.
Chicago can win now. Mirotic, Butler, McBuckets, Snell, and Rose fit great for him and the future is crazy bright with all those guys really young, except for Rose's knees (but Butler and Mirotic are still great building blocks)