Why don't teams want to give Shawn Marion a long-term contract?

People say that he's a problem in the locker room but I think he has more of a problem with the owner of the Heat not giving him a contract. He's one of the greatest and most versatile defenders in NBA History and if you don't think he's an all-time defender you're sleeping. I don't know who does the voting for the all-nba defensive teams but it's stupid that he's never been on that team. He's one of the few defenders that I've seen that makes the person he's guarding score less or shoot worse. He's very athletic and is a great rebounder. What the hell is wrong with these gm's that don't want to give a guy that averages 18,10, 2, 1 and 1 for his career? What do you think?