According to Dave Meltzer, the matches on Raw and Smackdown a few weeks ago, featuring Cena and Ziggler, were tryouts for Dolph Ziggler to possibly be moved up to the main events. Evidently, Ziggler passed the test and shall now face Edge at the Royal Rumble.
Source - EWN.
I'm so glad that WWE are trusting Ziggler, by moving him to the main event.. I'm shamed that Orton has been picked over Morrison, as I really feel that Morrison should have been on the card - He could have proved that he deserves to be there.