They've had so many opportunities in the last few years to tun him heel, and never done it. For some reason, Cena is a special case and seemingly exempt from the character change that every other top star has enjoyed throughout the history of WWE/WWF.
in the past it was a tease or had no reason for doing so,
For example, Nexus - They had no reason wwe needed cena as a face, the pg era was just kicked off in 2008-09, and wwe was extremely pg in 2010 and nexus was most likely because they had nothing to do that summer, also they made it obvious they he wasn't going to be heel.
exmbrace the hate - was just a tease that wwe made obvious to play people(if they wanted to turn him heel it would have been surprising)
wrestlemania 28 - it was miami, rock just came back no way cena was winning
Wrestlemania 29 - obvious from the promo, his life(in storyline) going down the drain, he wants redemption, he'll do anything, his attitude recently, the fact that people didn't notice it, the exact same promo ending as austin/rock 01 and the fact that wwe has no excuse not to unlike the past, it's obvious he's turning heel