There is a range of possible explanation but I prefer one that is simple - the move was more about getting on the England squad than everyone thought, including Defoe. After the WC, with things not going well and constant inquiries about coming back to PL, the project just didn't seem as compelling. Throw on top of that TL leaving, that was probably enough. He may have assumed that if MLSE got their money back in the transfer, they wouldn't care if he left.