Primarily, availability. IF you can walk into a Triumph dealership and get coolant than that may be the best option.
I dare you to try to get a single shim that fits everything from Bonneville's to Rockets from a dealership. They order everything- also why bikes sit in the shop for so long gathering dust. I tend to order parts from Hermy's (still a wait) and coolant (shipped) would be cost prohibitive.
Like motor oil, if it meets the specs IMO it's fine whether it says Triumph on the bottle or not.
