Because in the American cultural imagination the Vietnam War was a horrifying slaughter and a complete loss. But the nature of the loss depends on who you ask. Chuds say politicians hampered the generals, liberals will say lack of morale or something.

Vietnam is kinda unique in that most Americans at this point think about it through movies rather than an actual event. And movies always portray it in the negative, as a complete tragedy. So for modern Vietnam to be demonized you'd have to somehow portray what exists now in Vietnam as worse America's involvement and that's not possible