1) Vietnam broke the back of the draft system. In the aftermath of that war, after realizing force of law had been used to force Americans to kill and be killed in an invasion for no gain, there has not been the political will to reactivate the draft for any reason other than a direct military assault on home soil. Even after 9/11, calling up a draft for Afghanistan or Iraq would have been absolute political suicide for those who tried. The consequence of this in the American psyche is that soldiers who die in our foreign campaigns are "volunteers" and "knew what they signed up for," so it puts the consequences at arm's length; nobody carrying a rifle into a war zone with an American flag on their armor is doing it despite desperately not wanting to, and Americans are sort of fine with a self-feeding military-industrial complex that generally leaves the home front alone.
(This is, of course, not entirely accurate or true, but it's true enough that most Americans sleep at night while some of their children are stationed in Iraq).
2) The US hasn't lost any subsequent military engagement nearly as badly as it lost Vietnam. Afghanistan comes closest, but the 2,400 soldiers dead in over a decade of Afghanistan operation doesn't hit the psychic shock from 58,000 pressed servicemen dead in the 8 years of Vietnam. During the whole Afghanistan campaign, the US consistently lost more troops to injury and accident (across the whole service) than to enemy action; one doesn't get the same moral outrage of "bodies sent into the meat grinder" if most servicemembers dying while serving are dying in car crashes. And the fundamental shift of rules of engagement away from boots-on-the-ground and towards air supremacy and air-to-ground strikes lowered the body count to the point where far, far fewer Americans personally know a Johnny who didn't come home.