I suspect that body strength and engine size has a lot to do with it. The Prius has a very well engineered body structure, but I doubt if it was engineered to take a tow hitch. Also the engine is pretty small- 1.8 liters.
OTOH with a dry weight of 712 lbs and a GVWR of 1,286, it probably would work. Draw-Tite has a 2,000 lb, 200 lb tongue weight hitch.
David