I think because Vietnam was a war that happened in a different time and there were a lot of protests and bad publicity, such as Jane Fonda saying that soldiers were war criminals etc, and that they were lying about being tortured.
The previous wars were all about protecting your country, and part of the culture and psyche was about patriotism.
Whereas Vietnam came at a time, where there were a lot more different societal impacts.
Another thing to is, the culture in the previous wars was not to talk about how bad it was, move on. Whereas Vietnam was at a time where people spoke out and expressed their feelings about what happened.
And I think that is a good thing. Vietnam veterans are always seen to be like they never experienced war. They were subjected to a lot of crazy shit too.
Culturally with previous wars, men who were fucked up, just beat the fuck out of their wives and children and went down to the 'female free' bars and became alcoholics. Life was all about 'mens business' shutting up and covering up your feelings and emotions.