That is easy. Big Business didn't want to retool.
A far better question iswhy , after 30+ years is Canada still only half metric? ANd why do the metric bolt head sizesused in NA not use the same size wrenches common in Asia and Europe?
I worked in a totally metric manufacturing plant for years. Different sizes for different trades,mechanical and pipefitting were different over quite a few sizes for instance. I was told by a plant maintennance manager one time that metric washers were not made anywhere in the world, of course this person was a manager and not maintennance, the meeting ended in deafining laughter. Automotive uses a lot of 12mm as opposed to 13mm, 15mm and 16mm instead of 17mm or 19mm. The plant in question used every concievable thread standard on the planet as well even BSP.
