I think there are several reasons for that. John Nash was right, and MAD has been a factor in global issues since then.
But some other factors could also have some influence:
The aftermath of WWII did show the world that for a nation, engaging in an all out war was no longer profitable. Wars have been profitable for most of human history, but now, with such reliance on technology and logistics, which are so much easier to destroy than to rebuilt, wars are a serious economic risk, with total economic destruction a very possible result.
Also, for many leaders in prosper nations, declaring a war would be a cause of political death, as the majority of the population would be against it, and continuing with the conflict would require a dictatorship or other forms of destruction of democracy.
I think the economic argument is very strong, probably stronger than MAD, and the democracy argument can have a small positive or negative influence, depending on the society.
Unfortunately, there are still a few national leaders who don't entirely operate under those constraints. Fortunately, I think they're still militarily weaker than the big democracies (if the US still counts in 5 years), so deleting nukes might still be a good idea.
But some other factors could also have some influence:
The aftermath of WWII did show the world that for a nation, engaging in an all out war was no longer profitable. Wars have been profitable for most of human history, but now, with such reliance on technology and logistics, which are so much easier to destroy than to rebuilt, wars are a serious economic risk, with total economic destruction a very possible result.
Also, for many leaders in prosper nations, declaring a war would be a cause of political death, as the majority of the population would be against it, and continuing with the conflict would require a dictatorship or other forms of destruction of democracy.
I think the economic argument is very strong, probably stronger than MAD, and the democracy argument can have a small positive or negative influence, depending on the society.