South Korea is in an area of greater importance. For one is is close to Japan and in a area considered America's backyard. America could afford to lose in Vietnam in a way it never could or can in South Korea because of the importance it plays in the larger scheme of things.Again similar thing also being talked by US Political circle that they will not leave Vietnam and they will not leave Afghanistan.
Despite its many failings the South Vietnamese government in 1975 [as I pointed out previously] was in a far better and stronger position than the Kabul goverment was in 2021. That America decided to leave the country was largely due to there being no end in sight to the war and due to domestic American politics. Yes the Saigon government was weak and corrupt and these factors played a major role but again, so did a number of flawed and self defeating American policies.