I maintain the Vietnam erasure has a lot to do with it going directly going against the story, or if you like the myth, of America which is so incredibly deeply entrenched in both right and left. And the way generation after generation (on the Allied side) falls in love with WWII is because among so many other things, it's the ultimate good vs evil roleplay for them, it has the correct outcome, and any alternative outcome that's usually considered is inevitably a worse one.

Now you could say that the way civil disobedience and protests were also a good versus evil tale with the right outcome, but: Johnson, and even Nixon, don't function as well as the big bads. Both weren't dictators, they headed an elected, democractic government. And where WWII fits right in with the whole "Band of brothers"/"our brave men"/"support the troops" dogmas and credo, Vietnam has screwed up vets at best and My Lai at worst. Facing the complicitness of such a large part of the population, from the generals down to the avarage soldier, in the wrongs before a right could be achieved isn't something the US was/is used to.