As I see it:
There was a proposed nuclear deal back in late Clinton term, between North Korea and the United States.
North Koreans were rather willing to, since w/o Soviet Union, their prospectives were grim: Iraq has shown, just what true superpower can do if left unchecked.
But after rather shameful end of this term, it crumbled: Bush administration decided to walk away entirely, returning to kind of Cold War mindset "there is one Korea to talk with. Not yours."
Partially it was "new president" thing, partially based on assumption what North will just fall(Eastern block collapse wasn't that far in memory back then).
In any case, massive hunger was still recent memory, NK could actually be seen as crumbling, why talk&spend money on something, what you can just push down further?
This point probably was significantly influenced by South Korean intel&political community.
Thus we come to "axis of evil" policy. North was seen as something which will croumble by itself, even w/o any investement. This policy was half-assed for a long time, but stance toughened with time.
Problem is, North Korea actually survived.
And nuclear program was maintained by King Jong-il, as a way to force americans back to the table.
Contest of thickheads wasn't won by anyone in time, and NK just got Nuke.
Then came notorious detoriation of relationship between Koreas, which never was anything better than awful, but 2000s saw stream of economic cooperation. at least. Old Southern policies of "no backdoor for northern elites" were restated.
Here comes yong King Jong-Un, who(as i see it) decided in this situation to just go all in:
best way to get what they wish for(actual deal) isn't nuclear development, in threat of which few actually believed, but fact of stable and able nuclear triad, Carribean style: force your opponent to the table, whenever he wants it or not.
Backside of such plan - it's as provocative as possible, and provocative against opponent who's very capable of bringing Korea down. Plus, well, it's hard to implement, and is inherently dangerous to others(State which can't create truly stable and survivable command link is bound to create backup options. Basically it means to delegate launch permission to lower command levels under certain conditions).
