The armistice line was where the armies were. Trying to move it north would have been bloody, expensive, & not guaranteed to work.In today's media, South Korea are insisting that no attack on the North can take place without their permission.
Again the US is in a no win situation; dammed if they do and dammed if they don't.
What a shame that the Armistice Line was not agreed another 30 miles north making Seoul outside the range of the NK artillery. Of course it would still be within reach of missiles but at least it would have made some significant difference.
Any time after 10-07-1951 it would also have meant fighting over the place where armistice talks were taking place.