Contractual Purity: Cena has stated this as one of the main reasons why its difficult for him to turn heel. Cena has enjoyed the charity work that he has done as the top face of the WWE, becoming Make-A-Wish's most requested personality. He stated it would be work that would be hard for him to give up (and likewise the company if they didn't have a replacement ready) if he became a villain.
Money, Dear Boy: The reason for Cena's current face gimmick has lasted as long as it has. His merchandise alone has drawn in tons of money for WWE which has made them afraid of changing his character at all.
Going down with a serious injury literally days before he was to drop the WWE Championship at No Mercy 2007 to Randy Orton in what would most likely have been another intensifier to their already red-hot feud.
In similar fashion, his injury in late 2012 prevented us from seeing CM PUNK vs. JOHN CENA V (and the first four had all delivered for the most part) for the WWE Title, inside Hell In A Cell.