Because, North Koreans are dying every day from starvation resulting from the US-led sanctions.  North Korea treats the sanctions as an act of war.  They are smart and rational and will wait until they have amassed an overwhelming arsenal that can strike New York and Washington before demanding the end of sanctions, the removal of US troops from the region and the reunification of North and South Korea under Kim Jong Un.

We are rational and our response should be to let Kim Jong Un have what he wants because our being able to kill many North Koreans does not adequately compensate for the destruction of Los Angeles, San Francisco, Chicago, New York & Washington DC.

North Korea, as it develops its weapons, will also sell nukes to any country and/or terrorist group that opposes the US.  If North Korea avoids a war in the next year, then, one way or another, it is highly likely at least one of our major cities will be nuked in the next decade.