China is a super power. You know why? It's the largest world manufactory ever made! They have a billion people, atomic bomb and they can conquer the world! I don't know what thing exactly is making them stay calm.
A robber with a gun can have your wallet without even shotting at you. Why having a murder count if you can avoid it?
Many people that resort to robbing people do not have this rationale. If someone is willing to rob someone at gun point (still a very serious crime) then they probably do not understand the consequences of a murder or attempted murder change.
If a country is crazy enough to use it's nukes in order to try to get world policy changed then it's political leaders are likely immature enough to actually use them