A fundamental misconception
About war and killing. Sure, killing happens in war, but it is not what war is about. War is about making your enemy quit. Give up. No longer be a viable force, capable of expressing political will through force.
Pretty words to describe killing, but that is the point. Killing is just the most permanent and effective method. War is not about killing, it is about achieving specific political goals. When war is about killing the enemy, we call it something else. We call it genocide.
The rules of warfare came about in a time when wars were still primarily fought only between opposing military forces. Civilians were only involved when they happened to be on the ground being fought over. Pushed by the Europeans, codified in the Geneva and Hague accords, it was the last attempt to retain the concept of a "gentleman's war", a carryover from the ancient laws of chivalry.
With individual exceptions, soldiers, when shot, generally stop offensive actions. That is enough. Actually killing them is not needed when they are no longer capable of attack (or defense). So rules about weapons causing unnecessary suffering sound noble and good. And govts, engaging in "just" warfare, wish to be seen as noble and good.
On the pracitcal side, fmj feeds the best, costs the least, and always works as well as it works. JHP was, until fairly recently (in the history of ammo) not reliable enough for dependable performance. So JHP often functioned exactly the same as FMJ anyway, there was no percieved benefit to offset the additional cost. Particularly for the military, who has never considered the lives of our troops the paramount objective.
Our troops lives are improtant, make no mistake, but it is the mission that is most important. Lives will be spent to achieve that. The fewest possible is the goal, and we work hard to make that reality, but it is not the mission.
Since the military does not consider the handgun a combat weapon, adding JHP ammo to the supply inventory is not considered "cost effective".
The rules of the Geneva and Hague conventions only apply to signatory nations and their armed forces. They evolved in an era before the concept of total war that emerged through WW II. When it comes to small arms ammo, if fmj did not meet the military needs, we would not use it, treaty or not.
Just remember that the military's concept of what is needed and appropriate is not the same as our individual belief of what is needed and appropriate for our own personal protection.