Becuase the NK's never fully complied with the 1994 agreement. They never permitted the inspections they allegedly agreed to in the carter intervention.
By 1998, the NKs were already working on at least one other nuke weapon site and were no longer negotiating seriously regarding the 1994 agreement.
Chances are very high, imho, that the NKs already have a substantial arsenal of fission weapons, say 10- 12.
They may be well on their way to a fusion weapon or two.
This period of time is very similar to the time in the late 1930's where careful observation would see that Hitler was clearly aming towards war.
We have slept through the last 8 or so years.
We must now tread very delicately, since basically the population of South Korea is being held hostage to the tyrant of North Korea.
Ed