Absolute hammer to the national identity.
For a nation that has prosecuted so many wars before and since, it is quite interesting that this one was such a cultural blow.
Even Iraq, fought on the basis of fraudulent WMD claims, had no such effect.
Wild.
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.
Five years in jail is also quite a long time, and as the war dragged on I suspect a lot of folks weren't sure how long those jail sentences would ultimately be; remember, if you were drafted, dodged, were caught, and served, you'd be immediately drafted again and could go right back to jail (because the war lasted longer than 5 years). Since the goals were unclear, I suspect many served thinking the war would be over soon and a couple years in a battlefield with "the army that saved the world from Hitler" would be better than cooling your heels in jail for five years (and then being branded a coward by your family the rest of your life because your dad lost two brothers in his War and you were too chickenshit to live up to the family legacy).
Draft dodging wasn't the obvious correct choice until many years into the war; Americans had won (or at least come to a standstill on) drafted wars before.
I think shadowgovt's post was quite neat: the draft and the body count.
But the next war comes up, and everyone tells you that it's righteous too. Maybe they're mistaken or deluded. The people in charge, demanding this war, they aren't supervillains either. They believe they're fighting communism, and while they probably couldn't have known for sure at the time, history has shown it to have been horrible enough that some sort of strategy should've been employed (even if not a shooting war).
Are you going to be the one to back down? There's none of this wokeness that exists today. You'd be called worse than just a pussy for dodging it. Even those who knew of and understood shell shock only cut slack for those who had suffered it, you wouldn't get pre-emptive sympathy.
And those grandfathers and fathers who did go off and fight, you're sort of insulting them though not coming out and saying it outright. If you're morally justified in dodging the draft, should they have done the same? They didn't, so are they just amoral killers now? And this isn't going on in your own head, not entirely, you can tell they're sitting there thinking the same thing, wondering if you'll be one of the hippies that insults them by dodging.
There may be moral principles strong enough that it would demand someone dodge the draft, but those sorts of principles aren't popular or encouraged in the US. Why, if people will dodge the draft during bad wars because of this, what does it mean if the principles say "all wars in which enemies aren't landing on our beaching and dropping shells on our undefended cities are bad"? Well, it means that officeholders who will eventually want a war of their own can't have it. And all officeholders eventually want a war, even if they all can't agree on which.
That's why everyone didn't dodge the draft. Sociological pressures, psychological pressures. Ambiguous popular moralities and people not wise or strong enough to create better ones.
The US fought in a lot more than a half dozen wars in the century before Kent State and it takes pretty interesting morality to judge them all “more or less” morally justified.
Sure, there were many. Find me a highschool graduate in Podunk Alabama that knew all of those "more than half a dozen", and was enlightened enough to care that natives were genocided a century earlier.
"Gee, why didn't someone in 1968 have the same sensibilities that I have?!?!"
Many people were very proud of their service in WWII and Korea, and they passed that pride on to their kids. WWI was still in living memory at that time, too.
Kids were way less cynical. I think it's legitimately hard to imagine now.
I'm speculating, but I think that the few who refused to serve were met with the same outrage that someone like Weinstein receives today. It's no surprise that most eighteen-year-old boys could not do that, even if they were strongly against the war.