Because the team started looking like horsefeathers like they were completely checked out at times, plus all of the stories of how you had players going to management and saying they wanted him gone or supposedly even threatening to boycott if he was brought back: http://www.sbnation.com/nba/2015/5/28/8678683/bulls-players-tom-thibodeau-offseason-fired http://larrybrownsports.com/basketball/bulls-players-threatened-to-boycott-tom-thibodeau/263121 http://bullsbythehorns.com/bill-simmons-zach-lowe-discuss-behind-scenes-unrest-chicago/ I don't think it's too much of a stretch to think a coach like Thibs is going to have a relatively short shelf life. No matter how much a player initially buys in, most of them are going to be pretty fed up with a coach like that after a few years, especially if they don't pull off a championship or two. Bringing him back wouldn't have accomplished much outside of not having to pay for two coaches.