I think it's a new model in general. Adobe showed that middleware as a service is the way forwards. Crytek are doing this as well now with Cryengine. Epic also made UE3 available for free if you wanted to play with it, you only had to upgrade to the full licence if you produced something commercial from it.
Unity is good because it's easy to use and makes compiling for different platforms a snap. Most of the other serious engines don't scale well to mobile - X-Com for the iPad was made in UE3 but it's a bloated monster.