I believe the problem was that Americans had been led to believe that such an attack by the VC was impossible because we’d licked them so badly; the images of the embassy being attacked supposedly hit hard.
I’d think I’d been lied to as well if I watched the news from 1965 through 1968 and saw that; the inflated body counts and lack of real progress came home to roost with the Tet Offensive. If nothing else, it was clear that the ARVN couldn’t defend South Vietnam without massive US intervention/boots on the ground.
1972 was a different matter altogether, as it pitted conventional North Vietnamese forces against the ARVN with heavy US air support but no boots on the ground. While it was rebuffed, it did leave North Vietnam holding about a tenth of South Vietnam - and we agreed to leave them there as we signed our peace treaty. Thieu was furious, and understandably claimed the US had betrayed South Vietnam; I guess he just didn’t understand that political changes in the US would doom his country.