The vast majority of periphery nations simply cannot afford a nuclear deterrent without crazy high spending on the military, think 25%+ of GDP and an advanced knowledge and industrial base to support nuclear science. Such high military spending can easily lead to economic collapse. In contrast, nations with advanced economies and high GDP like Germany, South Korea and Japan could obtain nuclear weapons, build delivery systems, and maintain it all with minimal economic sacrifice by comparison, and already have the industrial and knowledge base to do so.

Nuclear proliferation globally would be one of the worst things to happen for the periphery. Rich countries would easily nuclearise, while poor nations would be left at an even greater military disadvantage. For every North Korea there'd be ten Frances.

North Korea is a unique case scenario. How many nations are led by the same family dynasty for decades who can plan the economy however they decide to, and have their main adversaries capital city in gun based artillery range? Not many.

I'd go onto say that the deterrence value of a small, almost purely nuclear deterrent without second strike capability is much lower than the average person thinks, or is portrayed in movies and TV. Seoul being in North Korean artillery range is a much bigger deal for deterrence than most think. And even with that conventional deterrence North Korea has, they still have to threaten nuclear war at very low levels of conflict to maintain deterrence. A few hours ago they fired off a bunch of shorter range hypersonic glide vehicles in a weapons test, after what happened in Venezuela.