Just a short history and technology note: First, the two Koreas created largely by the US after World War II – it was  largely a matter of our relative low interest in the region and the power of a growing China.
Secondly, as has long been quietly suspected,  North Korea is not a technology giant; and the missiles and atomic power they are advancing so rapidly as benefit it quite a bit from China. 
 The missiles that crashed into the sea near Japan, when retrieved by Japan and the US, had their major components made in China.