He's also a despicable sportsman, he's been out of form, he's 29 and not getting any younger, and by all accounts complete cancer in the locker room. Problems with Simeone, problems with Guardiola. We have a reputation for sorting out bad boys but why take the risk?
Also you must not have watched Bayern a whole lot. Robben, Ribery and Muller were instrumental, not Mandzukic. Gomez contributed as much as Mandzukic during that season with less than half the minutes played. If he was so instrumental they wouldn't have let him go so easily.