Instead of reducing the screen size (which Apple may not be able to do successfully), the body has been trimmed, giving users a device that is almost entirely made up of the display.
The other reason has to do with the status of theatrical exhibitors as the weakest link in the 3-D process. (Some theaters have already been caught reducing light levels on the screen to extend the life of expensive projector lamps.) What happened to me and others at the ArcLight the other night was the height of absurdity.