The threaded ring is heat treated to a higher surface hardness than the teeth are. Because having dissimilar hardness makes the surfaces less susceptible to galling. Take the shape of the mating teeth. There is clearance at the root of the tooth, and angled mating surfaces, so that if the teeth were to wear slightly, the coupled joint would be just as tight. Take the future.
Work is well under way for aluminum and titanium couplings.
Take the weight. Take the consistency. The couplings are manufactured in a completely automated process without human intervention. The beauty of that is that you end up with everything perfectly concentric. For one thing, they're now about a quarter of Bilenky's business.