He's a good young defensive player potentially available for nothing but cap space. Some team with money to burn and is looking to pick up a useful young player is going to throw a contract his way.
Kelly Olynyk is a "useful young player", and so was Jared Sullinger -- but I doubt anyone is giving them $20 million any time soon.
Noel also has the dubious distinction of being a mediocre rebounder, an offensive nonfactor, and and completely lacking positional versatility. Given that he's somewhat undersized, and generally not very productive, I see his ceiling as a system-type player a la Kendrick Perkins. Otherwise someone would have broken the bank for him already. Don't think it's an accident that he readily lost his starting spot this season.