It's just that a mission this large takes a lot of planning and coordination with allies and with China and Russia.
Here's what Stratfor (Jan 2017) has to say on the subject:
North Korea is powerless to prevent a U.S. strike on its nuclear program, but retaliation is well within its means. The significant military capability that North Korea has built up against South Korea is not advanced by Western standards, but there are practical ways Pyongyang could respond to aggression.BTW, a great deal of the planning has to do with what happens to North Korea after the fall of Kim.The North Korean military's most powerful tool is artillery. It cannot level Seoul as some reports have claimed, but it could do significant damage. Pyongyang risks deteriorating its forces by exposing them to return fire, however, which significantly restricts their use. Less conventional methods of retaliation, such as sabotage or cyber warfare, are less risky but also limit the shock that North Korea would desire.
After a strike, North Korea's most immediate and expected method of retaliation would center around conventional artillery. Many of the North's indirect fire systems are already located on or near the border with South Korea. By virtue of proximity and simplicity, these systems have a lower preparatory and response times than air assets, larger ballistic missiles or naval assets. Nevertheless, there are several critical limitations to their effectiveness.
Tube and Rocket Artillery
The biggest anticipated cost of a North Korean artillery barrage in response to an attack would be the at least partial destruction of Seoul. But the volume of fire that the North can direct against the South Korean capital is limited by some important factors. Of the vast artillery force deployed by the North along the border, only a small portion — Koksan 170-mm self-propelled guns, as well as 240-mm and 300-mm multiple launch rocket systems — are capable of actually reaching Seoul. Broadly speaking, the bulk of Pyongyang's artillery can reach only into the northern border area of South Korea or the northern outskirts of Seoul.
All forms of North Korean artillery have problems with volume and effectiveness of fire, but those issues are often more pronounced for the longer-range systems. Problems include the high malfunction rate of indigenous ammunition, poorly trained artillery crews, and a reluctance to expend critical artillery assets by exposing their positions.
Bkmrk.