Why Marco will stay a Bruin

Marco was one of the biggest surprises this year, in his first game, he did nothing with Montreal as a filler. However he agreed to play for 1 year with Boston and in his first game was great defensively, when Game 1 of the Sidney Cup final came around, he showed no change and continued great play. However, there is one problem, he is still contract-less and is a free agent after the cup is awarded. Boston obviously should resign him because Marco is their best defensive player, and basically only player who plays defense on the team. But why should Marco sign? Boston is a good team with a chance each year and Marco would possibly ignite a bidding war between the Habs and B's which my team will back out of meaning Marco wouldn't leave the Bears.