This loss had almost nothing to do with the military. When most people think we started to lose the war, the Tet offensive, was actually the knock out victory we needed to win Vietnman, but the reporters told only of the attrocities of the S. Vietnamese and pictures of Viet Cong running around the US Embassy, never did they tell that most of the territory conquered was regained within hours, it was a huge military victory for the US. The Vietcong were forced into hiding, 80% of their members were killed, captured, or wounded, but the public was lead to believe by the media that it was a loss for us, we could have ended the war in less than a year, instead it was forced to drag on for years, costing even more American lives. It was thanks to the hippies and draft dodgers that we lost the war. This war was not lost in Vietnam, it was lost at home, you would think that the National Guard killing 11 students at Kent State they would have taken the hint and shut up, letting the military do what they do best.
And a better name for this thread is "Why didn't the US win in Vietnam?" Because to ask "Why couldn't the US win in Vietnam?" is to say that a US victory was impossible.