He didn't ignore North Korea
When did the information become known about North Korea violating the 1994 and 1995 agreements? What was Bush's response?
Bush didn't go unilateral and try to invade North Korea. Instead, he worked to assemble several nations (China, Russia, Japan, and South Korea along with the US) into a six party set of negotiations. They tried to convince North Korea to abandon their nuclear program, offering inducements like resumption of food and fuel shipments.
The strategic and tactical situation in North Korea is different than in Iraq or Iran. It required a different response, which Bush provided.