I think Americans (and probably humans in general) have a distaste for local violence. Violence afar doesn't tickle the brain in the same way.

My ignorant take:

Media brought the horror of US casualties in Vietnam home in a mass and immediate way that didn't exist in prior conflicts. The novelty of that media combined with the casualty rates drove unpopularity. It made the violence feel more real.

Even if casualty rates in post-Vietnam conflicts were higher I'm not sure we'd see negative sentiment because media coverage of violence is so normalized now. Exposure to violence in media is no longer novel.