I'm not sure why; it is after all a very secretive dictatorship with almost no cross-border travel.(Original post by rsplaya)
It would be almost impossible for North Korea to develop nukes without anybody knowing. They would be annihilated if they tried anything.
But suppose PRC, SK and US learn about NK's weapons plans. They are then forced to start a conventional invasion. And since now US and PRC don't have nuclear weapons, there's not a lot stopping this war once the two sides meet up.
But did you bother to watch?