All the extra weight from the Shuttle's aerodynamic surfaces would really cut into SpaceX's cargo capability too much. And remember that the part that landed was only a small fraction of whole launch system's bulk. SpaceX is hoping to eventually get a quick turnaround which means not throwing away lots of parts. And in the short term they can make incremental progress because what they're using to land isn't much more than what they needed to get to space in the first place.