I'm surprised no one has mentioned the refugee problem yet.
Quite simply put, refugees from NK is one of China's biggest fears, and coddling a banana republic has always been preferable to having a warzone at their border. This, of course, hinges on NK not being a nuisance and inviting a war, which unfortunately they've not kept their end of the bargain.
Most of NK's most likely targets and neighbours, like Japan, South Korea and Taiwan, have a military pact with the US, meaning if they're attacked in any way, the US is obliged to assist. On the other hand China does not have a similar pact with NK as far as we know. Also China is more willing than not to avoid direct military confrontation with the US. So if North Korea decides it wants to go to war, it'll have the largest military in the world blasting it into oblivion. Meanwhile, China will be troops lined up at its border in Manchuria waiting to "discourage" refugees from crossing into China. Even if it doesn't do that, it definitely won't allow foreign troops in Chinese soil or offer any assistance to any party, NK included.