That image completely ignores the actual nuances of the situation that was the Vietnam war. First off it was in a very harsh unforgiving environment, its difficult to swing around your military might in a dense jungle that blocks tanks, swallows equipment and is extremely unfamiliar to you. However the farmers who had lived there their entire lives know about the tricks needed. They also didn't fight conventional warfare it was gorilla tactics.

And I think the biggest part the Free-Love and Anti-War movements started to go into full swing around this time. When your own citizens are demanding the war stop its hard to get any kind of real might and push behind it. The war faltered because the US simply didn't want to continue. Had they wanted to it could have easily steam rolled the Vietnamese fighters eventually. But it wasn't worth it and the citizens didn't want to continue either.