I can't help but wonder if the proprietary threads date back to when bigger cities had multiple fire companies operating in different neighborhoods.
Remember, back in "the day" fire companies used to fight over who would put out the fire, since the insurance companies paid the first folks to 'water' the fire. Maybe the modern mindset was "NY water for NYFD only", "Boston water for Boston FD", etc.
Also, as anyone who's worked for a municipality can tell you, once the wheels start turning, it's hard as hell to change their direction. Once all the engines got hoses with thread 'x', it was "cheaper" to keep special ordering new hydrants with thread 'x' than:
a) get all new 'spuds' (the threaded part of the hydrant port) for all of the city hydrants, and pay the water guys to install them, and
b) get all new couplings for the engines and hoses.
