I have a couple of theories to why this might be.

The first is that the Vietnam War was in a time where it was publicly acceptable to denounce the war. During WW2, the nation was united under one patriot banner to defeat evil. It was clear cut, Nazis are bad and they must be stopped. Having the national media draw attention to PTSD (back then it was called "Shell shock" or "Battle fatigue") was not in the nations best interest at the time or would not have been as accepted in general conversation.

The second is simply that as communication technology gets better, the public has a better understanding of the harsh realities of war. Even pictures taken of dead solders in the Civil War threw many Americans off.