The domestic situation in the US meant that successive Presidents had their hands tied and could not dedicate the necessary resources to the war effort.
The Vietnam War was unwinnable from the start.
The Northern Vietnamese communists employed highly effective military strategies to defeat the US and South Vietnamese.
The communists didn't win the war, the US lost it due to extensive flaws within its war strategy.
The South Vietnamese Government should have done more to help with the war effort. It was their country, their civil war, and ultimately responsibility for defending it must fall on their shoulders.