Do you understand the point of the PPV business? The point is to get people to buy the PPV. By turning on Wyatt now, you sell an encounter at the Royal Rumble. Why would the WWE give away a money producing storyline at a show people have already paid for? PPVs are meant to be the payoff of a storyline, not the beginning of it.
While I agree with Sly here... I don't think you need that match at the Rumble. The logic is perfect, but let's be honest here... the Rumble sells itself. IMO, Wyatt/Bryan isn't a match that's going to make someone that wasn't getting the Rumble... get the Rumble. I can barely name a relevant non-title singles match that has taken place at the Rumble in 20 years. This will be the same. Will it be a great match? Hell yes it will. I'm attending so I'm excited I get to see this. But it's just not something that is going to garner a significant number of PPV buys in itself. Now if you want to announce Jake the Snake is going to be in the Rumble, or Hogan is going to do some sort of appearance, that will sell PPV buys. Those are "need to see" moments. But when someone can pay $9.99 to watch it via WWE Network next month, I don't Wyatt/Bryan turning it into a "must see live" PPV.