The Celtics put a lot of emphasis on analytics when making their decisions. Stevens is probably the coach most comfortable incorporating these factors into his gameplans. I believe statistical wunderkind Drew Cannon is following him to Boston.
With a 6-year contract and lowered expectations due to the roster, Stevens isn't under any pressure to win right away. Actually, it would probably be better if they were bad next season for the draft pick.
Instead of hiring another NBA retread, the Celtics are taking a risk. But this team isn't winning a title soon anyways, so it's not like he'd be hurting the chances of contending. The upside of Stevens dwarfs that of any other realistic candidates.