Last I checked, no one dared to attack them before they had nukes because of China's promise (made good in 1950) to use their military to defend the regime in Pyongyang, and the massive array of conventional artillery pointing at Seoul just across the DMZ, where 25% of South Korea's population resides. Also: https://news.ycombinator.com/item?id=47192139
Current stance on negotiations is a miscalculation IMHO, they likely wanted for negotiations to drag on for a long time.