The obvious reason is for deterrence of an attack or invasion from a U.S. seeking regime change. However, military action by the U.S. was already extremely unlikely as any such action would put Seoul, South Korea’s capital, in danger of being hit by the thousands of artillery pieces just north of the border and well within range. That’s aside from the U.S. being overextend[ed] in Iraq. So a nuclear deterrent is only another level of deterrence.
The not so obvious reason is that North Korea has been implementing a strategy of disengagement since 4 October 2002, when then U.S. Assistant Secretary of State James Kelly [was] in Pyongyang meeting North Korean Deputy Foreign Minster Kang Seok-Ju. When confronted with U.S. evidence, Kang admitted that North Korea had secretly continued a nuclear-weapons development program.
After that the words “complete, verifiable, and irreversible” became a part of the U.S. negotiating lexicon concerning denuclearization, which caused a shift in North Korean strategy from Regime survival by Extortion of Concessions to Regime survival by Strategic Disengagement.
North Korea cannot accept engagement for two primary reasons. First, invasive inspections would make the regime look weak internally and risk control of the military. Second, inspections on the scale that would be required for any new package deal would likely bring in an unprecedented influx of foreigners, something North Korea does not want.
This is because the legitimacy of the regime is build on a cult mythology that would be in jeopardy if outside information were to reach the isolated and misinformed North Korea population. The exposure of the North Korean people to reality vis-à-vis the cult is an enormous vulnerability for the regime.