Why are bearing ball sizes measured in imperial units? Most everything on a modern bicycle is built to metric measurements - see, for example this question. Major exceptions include bearing balls, where metric balls don't even merit mention on Sheldon's site. This results in some wacky mixed engineering, like cones that are precision manufactured to metric measurements, apart from a race curve that is precision manufactured to an imperial radius.

What is the history behind this? Why wasn't there a Continental European standard that caught on after metric became almost universal? Even French bikes from the seventies, which have metric versions of everything else, use imperial bearing balls. I guess the same question could be asked about the pedal pitch mentioned this question, but I'm guessing that's because the Brits came up with reverse threading first.
My father worked at Fafnir Bearings for 35 years and even though it ceased to exist as an independent company about 50 years ago,  the trademark is still active.  In the post World War 2 era, Japan rebuilt its bearing industry using Imperial standards.  Bearings are a strategic technology in a manufacturing society and any rational government will promote policies that protects and promotes important  sectors.  At the end of WW 2, the US bearing industry had the right processes to make the best bearings.  They had the “recipe”.  Bearing plants in Japan and Germany were prime bombing targets and their assets, including people were very much destroyed. This made US and to some extent English bearings the target to reverse engineer after the war. If you need to figure out someones recipe, you should stick to making the same thing.  Annealing times and temperatures are dependent on the dimensions of steel.  Automotive and bike bearings were nearly optimal at a low price point, in the 1950’s .  The machines and tooling were well established, and an export driven economy would want to produce standard products.  

I think it simply is a matter that making these sorts of bearings didn’t merit the resources to shift to a different standard.

Another factor is that as America has deindustrialized, the machines and tooling were sold off to Asian  countries.