Source: The Wrestling Observer Newsletter
According to sources, WWE and 2K Sports chose John Cena for the cover of WWE 2K15 in order to sell more copies. Cena was picked several months ago, before Daniel Bryan won the WWE World Title at WrestleMania 30, and the feeling was that he is their most well-known guy so he will help sell the most games.
Cena has never had a solo cover for a WWE game and the belief is that even those who dislike Cena will still buy the game, while casual fans will recognize him and be more likely to pick it up based on that.