European nations have nuclear neighbors (France, UK) they can rely on if need be. Japan has the US to shield it from China (even still China pushes them around a little) and both China and the US to shield it from NK. South Africa is the dominant power in the region (Zimbabwe won't be annexing them anytime soon) and they have friends that like intervening in matters on the other side of the world (US, UK).
Ain't nobody with half a brain giving up their nukes after what happened to Libya. I'm surprised that Iran is disarming as much as it supposedly is. Quite frankly I'd like to see more proliferation among smaller nations because it keeps regional powers from acting like jerks. Look at how well nuclear weapons have kept the peace between India and Pakistan. Would Russia would have pushed around a nuclear Afghanistan, Georgia or Ukraine? Would Iraq picked a fight with a nuclear Iran or Kuwait? Would the US have invaded a nuclear Iraq?
Nuclear disarmament is a lot like gun control. The people who have reason to feel the most secure tend to be the first to tell everyone else to give up their arms and the guy in a neighborhood the police respond to "when they get around to it" who's upstairs neighbors are probably cooking meth wonders what kind of lunatic wouldn't want the ability to defend themselves if the need arose.
Your proposal to promote peace through nuclear proliferation would probably work great for a while. Then twenty or fifty or a hundred years down the line, something will go wrong and civilization will die. That doesn’t seem like a good tradeoff to me.
South Africa only did it because the right-wing white minority regime knew it was falling to internal opposition that was not only racially different but perceived to be of diametrically opposed ideological and, at least potentially, geopolitical orientation, and didn't want to supply nuclear arms to a hostile group. I suppose if the British government expected to fall to, say, a domestic Chinese-aligned quasi-Maoist opposition and had sufficient lead time to do so, it might disarm, too.
If you don't know why most nations don't voluntarily disarm in any other circumstances, the example of one of the few that did—Ukraine—might clear that up.
All four of these countries are under the US's nuclear umbrella and so don't need nuclear weapons. Which suits their politicians just fine: they get to have the safety of a nuclear deterrent without having to take the PR hit of supporting a nuclear weapons program.
But take note of groby_b's comment that America's commitment to the nuclear shield is suddenly looking very shaky. While they're still very much the minority for the moment, a few politicians on the Japanese right are gently bringing up the possibility of Japanese nuclear weapons, and if Trump keeps snuggling up to Putin, it wouldn't surprise me at all if Germany, Italy and Spain started seeing the same thing.
Can't they just snuggle up to their close neighbor and EU/NATO partner France? France is a nuclear power too, don't forget. UK is one too, and still part of NATO.
