Because China said that if the US shoots first, they would get involved in the fight (helping NK). If NK shoots first, China will remain neutral.
Both the US and NK want the other side to fire the first shot.
Anyway, Trump's squeezing the regime effectively and I think the Kim problem will be resolved internally before he can start any war.
I found this post to be logical:
Cementjungle:
“Kim is begging to be attacked. Why? Because China said that if the US shoots first, they would get involved in the fight (helping NK). If NK shoots first, China will remain neutral.
“Both the US and NK want the other side to fire the first shot.”
[unquote]
[And a stalemate works against the one who fails to thrive economically.]