Real reason is just the WWE being too afraid to actually turn Cena heel because they are that scared of his merch sales going down. One of the really bad side effects of revolving the company solely around one guy for over a decade while ignoring building up others to be his equal or near equal in status.
Him being the arrogant gatekeeper and constant shrug of his opponents being worthless would work way better if he was a heel. Especially when his opponents keep bringing up valid points against Cena and his antics.