I think it's also to alleviate the burden for developers to maintain two separate apps. iOS7 design conventions drastically differ from iOS6, so keeping both up

I think it's also to alleviate the burden for developers to maintain two separate apps. iOS7 design conventions drastically differ from iOS6, so keeping both up to date within the same app will be a pain due to legacy code. If the backend doesn't change, this makes it easier to move forth with a new design.