The reason for this is arguably because we could not. North Korea borders both China and what was at the time the Soviet Union. The cliff notes of the Korean Wa

The reason for this is arguably because we could not. North Korea borders both China and what was at the time the Soviet Union. The cliff notes of the Korean War (at least after the point of division and active war) are that North Korea decided to invade South Korea. They had South Korea on the brink of destruction and then the UN/US intervened on behalf of South Korea. The US/UN enforces then pushed the North Korean forces back north and to the brink of destruction. Then Chinese (and to a much lesser degree Soviet) forces intervened on behalf of North Korea and pushed US/UN forces back beginning a sort of see-saw that ended up with neither coalition able to make meaningful progress. After some time of a back and forth, an armistice was signed. I think it's somewhat unclear how the balance of powers changed in the time after this.