>>64148154
Because of NATO, which only standardized it as an afterthought because the krauts left stockpiles of it all over Belgium and France in the 1940's. Handguns were already becoming obsolete in a military context before WWII was even over due to cavalry going the way of the dinosaur. Sidearms were basically only carried by MPs, paratroopers, machine gunners, tank crews, and officers. The average infantryman was not issued a sidearm. 32 ACP was still the most common cartridge carried by European police in those days, so 9mm was seen as good enough because NATO would have mainly been using it in submachine guns anyway.
