Because China thinks they are the big kid on the block and won’t respond well to ultimatums from anyone including us. They know our dear leader lacks the stones to do anything.
And no China doesn’t care about a nuclear Japan. NK is building a bomb for America, not for anyone else.
Forgetting Obama, they would have no other way to respond. "Like it or lump it" being the catch phrase. A big part of their keeping "big kid" status is Japan remaining non-nuclear. I'm quite certain they care about Japan going nuclear.
China has a great deal of influence over NK. While NK is willing to starve millions and is thus resistant to pressure from the outside world, they are not capable of withstand being cut off from China.
China may like to think of itself as immune to pressure, but its not. Japan tying nuclear capability to NK force their hand.
The trick would be getting the Japanese on board to just do it, no threat, no ultimatum, just an announcement that it will happen. Giving China the alternative through backdoor channels would allow them to save face. They would act, then Japan would decide they no longer needed to go nuclear.
China supports NK because it is a convenient proxy to hit our pressure points. The natural answer would be to make NK a bigger pain for China than they are for us.