The feeling within the company is that while Miz is a good antagonist, they had done as much as they could with him right now in that role.
Since Miz comes off so well spoken on appearances and is willing to devote a lot of time to promoting the company, it made more sense to put him into a babyface role that would better suit that.
The company's interest in Miz appearing in Studios projects is also part of the reason. He's slated to star in their ABC Family 2013 Christmas season film and filed the latest "Marine" film earlier this year, and it's felt both projects would resonate better with the audience if Miz is in a babyface role.
The switch to babyface also gives him a slew of new names to work with, including CM Punk, Dolph Ziggler, Alberto Del Rio and Antonio Cesaro, among others. Miz vs. Ziggler will headline the 11/21 Main Event episode.