>>63941637
Poverty.

The Americans and other countries spent as little money as possible on their militaries, leaving them with outdated equipment, untrained troops, and amateur leadership. Countries that actually had money absolutely transitioned to more effective firearms. The Dreyse predates the 1861 in your picture by twenty years. By the time of the civil war, both the British and French were also quickly abandoning muzzle loading rifles in favor of breech loaders.