#1 There's already another John Cena in the roster
Not so long ago, we saw a feud between John Cena and Roman Reigns, which ended with Cena passing the torch to Reigns, making him the brand new 'guy' in WWE. Roman Reigns has been a very good representative for the company ever since and is a real-life superhero to kids worldwide.
Since the torch has already been passed to Roman Reigns, it does not really make sense for John Cena to come back and assume the same role that The Big Dog has been playing so well for so many years now. And hence, a heel turn is the right way to go about things, in my opinion.
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In fact, one could go so far as to say that someone like Drew McIntyre is being groomed to take on the same position and become a similar representative for the company going forward, which means that, in not much time from now, he will be yet another John Cena making it a sum total of 3 in the company.