As I've mentioned before, I lean to John Mueller's argument that the seeds of the decline in interstate war, or at least in major-power war, were sown in 1914-1918. I'm not saying this is the whole story re the decline, but I think it's part of it.
Here's a passage from Mueller's Retreat from Doomsday (1989), pp.55-56:
That World War I was a watershed event in attitudes toward war in the developed world is clear. Exactly why is less clear.... The impact on war attitudes of the Great War's physical devastation and of its horrifying weaponry should not be discounted.... But the bone-deep revulsion it so widely inspired and and the very substantial blow it administered to the war spirit so prevalent just a few years earlier should be credited at least in part to the insidious [I might have chosen a different word] propagandistic efforts of the prewar peace movement. The war proved to be a colossal confirmation of its gadfly arguments about the repulsiveness, immorality, and futility of war and of its uncivilized nature. Of course, the war also shattered the peace movement's airy optimism, and it certainly undercut its proposition that Europe was becoming progressively more civilized; but that was nothing compared to what it did to the notion that war was progressive -- as well as glorious, manly, and beneficial.... Since the peacemakers of 1918 were substantially convinced that the institution of war must be controlled or eradicated, they tried to apply some of the devices and approaches the peace movement had long been advocating.He continues:
For reasons that seem in reflection to have been special, it didn't work out so well. In Germany a leader arose who almost single-handedly brought major war to Europe, while Japan, a country that had not substantially participated in World War I nor learned its lessons, set itself on a collision course in Asia that was to lead to national cataclysm.If one accepts this narrative and explanation, the UN Charter formalized a change in attitudes that had been well underway for more than two decades, which could partly explain why the trend line of decline in interstate war does not track neatly with the UN Charter's adoption.
ETA: If Mueller is right, an underlying normative evolution is mainly responsible for the decline of major war in the 'developed' world, rather than the use-of-force rules themselves. Whether the argument can be extended to cover the decline of interstate war in general is something one could debate.
Note: Edited after posting to fix a grammatically challenged sentence.