Would imagine with the Euro's next summer he wants to make sure he is playing every week hence him staying at Villa rather than to be a squad man at Man City.
I reckon if he has even a half decent season this year, next summer he will move to a big club after the Euro's.
Fair play to him though, it would appear he has put football before money (i am sure he isnt short of a few quid anyway) and Villa did seem to look after him when he barely played for a couple of years due to injury so maybe he is repaying them because without him based on what they currently have (assuming Benteke goes and how they finished last season) i think they are in for a hard season.