I think the reason we don't have a standard is installed infrastructure. When an area has hundreds or thousands of existing hydrants and miles of fire hose, conversion just is not economically feasable. I worked with a volunteer department near Denver and we had taps and dies to convert to the Denver standard. For cost reasons, we bought equipment anywhere we could find a good price. The thread diameter was standard but the pitch was slightly different. Since fire protection in the US dates back 200 years or so, communication was poor and each area probably just set a standard as the need arose.
