Stephen Jackson was really, really bad last season. And he was a disturbance in the Spurs locker room, complaining about his minutes, which eventually got him fired.
We don't need any of that here on a rebuilding team. Furthermore even if he played better and didn't complain, I'm not sure that he is the kind of veteran, I want around our team. He probably wouldn't really care about the Celtics, and might give our new coach a very hard time.