Let's use a fancy word. Paradigm. Slideshow games are a paradigm, a way of doing things. Limiting of course - yes, no unrestrained 3D movement. That's an insuperable problem if you want a 2D game to be 3D, like it would be an insuperable problem to watching a B/W film if you wanted it to be in colour. But sometimes having limitations, having a set framework, is an advantage. It means authors can concentrate on writing, and not worry about the technology. It means players know what to expect, perhaps it won't be leading-edge/bleeding-edge, but many will be happy with that.