Perhaps it might be better to look at it from the other direction. Why did so many countries in the late 19th c. want to drop their use of ammunition of around 11mm- .45 caliber? That one is pretty straightforward: smokeless powder gave higher velocities than black powder, so cartridges with smaller, lighter bullets were possible, giving more cartridges per kilo. How much smaller became the next question: heavier bullets could have a longer range, but heavier ammunition was more trouble to transport, and the point of compromise between these varied. Italy, Japan and Sweden would adopt 6.5 mm, at the small end, and France and Germany would adopt 8mm, at the other. ( The US Navy even had the 6mm Lee, but it was little more than an experiment). Some countries changed their minds, as well. Japan would drop its 6.5 Arisaska for a 7.7mm cartridge, and Italy its 6.5 for a 7.38 mm cartridge, for WWII.