It would require making the engine support the 2d graphics which would be quite a bit of work. Also, new graphics would have to be made for every single unit because 3d models could not be used if you wanted a 2d game engine, and that would require more work, plus, it would greatly increase the size of the game (hard disk space) as the animations for each unit take up space. It is possible, though, to include lower polygon models that require not as much in a computer, and I believe that they are making lower polygon models as well. this way, the computer would not have to render as much at a time.