A totally genuine question - why are prong collars the norm in the US?
I would not consider prongs to be the norm in the US. If one is looking at a specific population of people, such as those who might be found on a GSD board where people own a large breed and are for the most part more educated than the typical pet owning public, and also more inclined to do training, including training to a higher degree, then they may seem to have a large following. But as far as most pet owners in the US, they certainly aren't the norm.
If your dog wears a prong, why did you come to the decision to use one in the first place?
Many people use the prong to give them "power steering" over a dog who may physically outmatch them. Certainly the dog can be trained to a level where a prong is no longer needed, but many people don't have the time or inclination to put that much training into the dog when they have access to a tool that can provide sufficient control to manage the dog and keep everyone safe. Really the same reason bits are used to ride horses... humans need some form of mechanical leverage to keep control.
From a training standpoint, as opposed to a management/control standpoint, they are one of the most versatile tools out there and this is why they are one of the most popular amongst those who do high level training. Not just for large dogs either. They are often used on small dogs as well because here it isn't about physical control but rather about clear communication with the dog.
A properly sized prong collar in the hands of someone who knows how to use it is extremely versatile. It can issue very light, subtle signals to the dog for things such as teaching the dog to yield to light leash pressure for fine tuning positions and body mechanics. And for major offenses it can issue a severe enough correction that even the largest, hardest, highest drive dog is going to notice. It is a tool that makes it easy for the trainer to tailor the strengh of the correction anywhere from very light to very hard, depending on what is appropriate for the temperament of the dog and the situation that earned the correction. Depending on how the leash is attached the force can be distributed around the entire neck, or in a directional manner, both of which have advantages for different exercises.
Due to the action of the collar, far less physical energy on behalf of the trainer is required compared to most other collars. That means better timing of the correction, less telegraphing of the correction to the dog, and less extraneous movement of the trainer disturbing the dog. All of which helps keep communication with the dog more clear and more simple cause/effect with no emotional baggage or other disturbance associated which is as it should be with corrections.
It can also be used in completely non-corrective ways as a device for stimulating drive and energy.
It is safer than a traditional choke chain as it is less likely to cause damage to the trachea and is impossible to actually choke the dog. Corrections with a prong tend to be little more than a flick of the wrist and thus involve much less yanking than with other collars so the risk of spinal damage is less. From the dogs' standpoint, I believe they are more humane. The pinching action is something that dogs naturally understand as being a correction. They learned that one in the whelping box. Whereas being yanked about or strangled aren't something that dogs naturally understand as corrections. Being nipped on the neck, yes. Having air cut off, no. So the dog has less of a learning curve to get over to understand what the collar means.