The NK regimes number one goal is to stay in power. Attacking the US mainland (especially with a nuke) would be about the quickest way for that regime to be obliterated.
All it takes is one crazy person in the right place and time to hit the button. Unless you can guarantee the regime builds in a robust failsafe system requiring authorization from multiple people... usually how dictatorships work, the dictator demands the ability to act without even secondary authorization.