Before the Anti-Miz faction go mad, let me explain. In no way do I mean he is some sort of Cena-beater or the catalyst for his downfall. What I mean is he is the exact opposite of WWE's flagship face.
Where as Cena is the near unbeatable, unstoppable, courageous, all-in-all good guy of the company, Miz represents the polar end of the mould. Miz is very much the type of wrestler that any man could defeat in a match.
This leaves the WWE Championship open to all challengers.
With Cena as champion, only a few men could realistically take the title away from him. With Miz, guys with the stature of Evan Bourne to The Great Khali are possible contenders. Once again, this is not to say WWE would have Miz lose to anyone, but the possible drama is there when needed.
Also, if you put your personal feelings aside, what do most of the WWE fans want to see Cena do? The majority of his fans (whether kids, women or men) want their hero to win and regain/retain the championship. Since the majority of fans would pay to see Cena be successful, those fans will also pay the same amount of money to see The Miz do the exact opposite.
They will pay to see if he will get beaten up, lose the championship or be humiliated. In a respect, WWE could pull in just as many ticket sales from this man's hatred as they could for Cena's support.
Not since Chris Jericho's 2001-02 reign has a champion had the potential to gain heat from just holding on to the belt after a close-call match.