There were many tunnels in this region and most of them had anti-nuke quality with enough ammunition and food. American, Japanese and South Korean did not expect to meet another Vietnam War here. AJSK allied forces wanted to finish the war quickly but could not use many weapons. Certainly this war was exceptionally cruel. American bombs could level the mountains.
American missiles penetrate mountain caves as they had eyes.
The China-North Korean troops paid deadly high life cost. There was no way for the Americans to bomb the whole mountainous North Korea into a plain. If they wanted a victory, they eventually got to recover caves one by one, recover mountains one by one. This would have to depend on traditional warfare. But when American soldiers got out of planes, helicopters and tanks, they would indeed by so-so.