When George W. Bush came into office, North Korea had maybe one nuclear weapon and verifiably wasn't producing any more.
To say that the United States has pursued diplomacy with North Korea is a little bit misleading. It did under the Clinton administration, though neither side completely lived up to their obligations. Clinton didn't do what was promised, nor did North Korea, but they were making progress.
In 1993, Israel and North Korea were moving towards an agreement in which North Korea would stop sending any missiles or military technology to the Middle East and Israel would recognize that country. President Clinton intervened and blocked it.
If North Korea doesn't have a deterrent, they will be wiped out.
In any case, it is better to have some deal than no deal, but it's interesting that Obama picked the day of implementing of Iran deal to impose new sanctions on North Korea.
I don't think that the United States cares. They just assume that North Korea will soon have nuclear weapons.