Why, exactly? I can't imagine that it's a wise move strategically, from a North Korean perspective. Denuclearisation seems absurd when your opponent isn't doing it.
Sanctions? The threat of being obliterated by a massive opponent? Pressure from China?
They've been pretty good at ignoring that for the past decades
They are cumulative though, and the efforts have been ramping up.
Trump?
No doubt. Historical examples are not good. The nuclear club gives you clout.