If North Korea falls it's reconstruction will most likely involve a 1 state solution. Unless South Korea's government falls at the same then the current South Korean government will be in control of all of Korea. China doesn't want a pro US country sharing a border. This isn't a keep the US happy because they are China's biggest trading partner question. This is a what if China and US go to war question. Obviously I don't think China and US are going to war anytime soon, but you don't play international politics without thinking out all the possibilities.

It's like the Cuban missile crisis. The US wasn't at war with Russia or Cuba at the time, but IF it did happen, we sure as shit didn't want missile bases in Cuba. Same thing here, IF it did ever happen, having a pro US country that allows US military bases in their country sharing a border with them is bad.

Now I will admit that China might change it's mind, because NK missiles are getting better and pissing people off and Trump is a crazy fucker who will need a even bigger distraction soon, but that has been their reasoning for supporting NK since the Korean War.