A number of reasons were cited, including his inability to share the court successfully with fellow big man, Joakim Noah. In fact, that might be the key factor in the decision.
Now, more from K.C. Johnson:
…the biggest reason Pau Gasol and the Bulls appear headed for a split in free agency has less to do with Gasol and more to do with Joakim Noah.
Though front-office meetings have yet to finalize the Bulls’ Plan A for this offseason, there is strong internal desire to re-sign Noah on a short-term deal. Noah long has been a favorite player and ambassador of Chairman Jerry Reinsdorf. Executive vice president John Paxson drafted Noah, and their bond is genuine.
Johnson also stated that Noah's importance to the franchise was amplified when he expressed anger with both of the Bulls' leaders, Derrick Rose and Jimmy Butler, for not speaking up at a team meeting earlier this month.
Noah and Gasol both want to start, but both can't in this system. Chicago doesn't have a shortage of big men with Taj Gibson, Nikola Mirotic, and Bobby Portis, and both Mirotic and Portis have the ability to stretch the floor, opposed to the two veterans in question.
Chicago's front office is certainly fond of Gasol, but they're much more confident in Noah's precscence in the locker room and his ability to instill that tough defensive mentality that has escaped them in head coach Fred Hoiberg's first season.
Noah doesn't exactly fit into Hoiberg's fast-tempo style, but with all of the chemistry issues they've endured this year, retaining Noah might be essential to the Bulls bouncing back in 2016-17.