https://rusi.org/commentary/north-korea-2009
Because they don't care and Kim certainly doesn't care.
Asking China to do something is like the rest of the world asking Canada to sort out the USA's issues or vice versa. It's not happening.
North Koreans have suffered hardships for generations, all sanctions do is justify their governments stance that they are under attack from the USA, they have not helped issues only added to them.
Lets say USA goes in an bombs every military base they have, do you really think they will wave a white flag? Or do you think they will just rebuild with newer technology and be consumed with paying America back.. cos they will. That would be the one act that would justify everything Kim and others have warned about to North Koreans.
At some stage the world will have to listen to what Kim wants, and then work out a way to move forward. Certainly the USA and Japan need to get the fuck out of Kims face.