score:19
First of all, armistice was supposed to be a temporary measure on the way to an actual peace treaty, it was not expected to last as long as it did. But at the moment of its proposal it was not accepted by South Korea - or by North Korea, for that matter. In fact, USA officials first started talking about peaceful resolution in December 1950, after China's intervention made it clear war isn't ending anytime soon, but until hostilities reached an obvious stalemate in June 1951 both Koreas wanted to fight until all of the country was united. US and USSR pressed for armistice, and in 1953 the demarcation line was drawn where the forces happened to stand at the moment - both sides just withdrew their forces 2km back from the frontline.
At that time, North Korea actually didn't have any artillery capable of hitting Seoul from that distance - so this possibility didn't enter armistice designers' minds, after all, if a peace treaty is to be accepted, it would include an agreement on borderlines which would probably go along the 38th parallel, taking the city out of the danger zone. But no such agreement was reached, and the temporary demarcation line became a de-facto permanent border.