Rottweilers are used. You can see one in the pic I provided of Royal Marines using war dogs. To my knowledge Staffords are not by any force.
Why are German Shepards overwhelmingly used by more than any other breed? I heard it from another trainer this way, and I think it answers that question best. He said that there are dogs bigger and more powerful than German Shepards, there are dogs that can scent and track better than them, and there are even some who are smarter such as Border Collies, but there are virtually no dogs that can do it all so well in one dog. Only the German Shepard's very close cousins, the Belgian Malinois, Rottweiler, and the Doberman Pinscher can do so well in all those fields. I suspect that only numbers put German Shepards above their cousins as it is the most popular breed in the world. They are certainly my favorite.