Well the first thing preventing it is that presumably none of these countries want to have a nuclear bullseye painted on them for literally no real benefit.
Let’s see, should I host some WMD for an unpredictable dictator halfway around the world, isolate my country from trade with most of my neighbors and make myself a first strike target in a nuclear war…. OR should I stay the fuck out of this mess and just say “No thank you” and continue my regime as it is.
Yeah Putin may want to position nuclear weapons in the Western Hemisphere but why would anyone want them?