Mostly because he has nowhere left to go as a heel, after jobbing to all the headliners in convincing fashion since they started to punish him. I just can't see Vince have a former WWE champion stay in the background or midcard hell forever. Especially one willing to make lots of appearances for the company.
So the only way forward is to turn when he returns from making his movie. With his not exactly intimidating face and body, Miz might be better suited to against-all-odds babyface role. He has the promo skills to get over and the clean shaven babyface look, not to mention the 'really' thing sort of got the crowd on his side.
They also need another houseshow working face headliner, after losing 'Taker, Triple H, Shawn Michaels, Jeff Hardy and Edge in the last 2 & 1/2 years. They've sort of lost Rey Mysterio through injuries and now drug issues; Chris Jericho seems content playing heel and the heel scene is brimming (e.g. Lesnar, Del Rio, Bryan, Barrett, Rhodes and the recent arrivals) .