Two reasons for me. Firstly, he didn't play his best until the last 6 months of his contract. Secondly, after his enforced absences from regular first team action, you can't be sure how that will affect a guy's confidence or form. A third resason (at no extra cost) is the potential abuse he ws likely to take (as per Hollis, Vigurs, Lawson etc.) from some fans for not being exactly like the guy he replaced.
Having said that, I wasn't expecting him to be mince - but was not expecting a 20+ season from him.