Because we didn't end the war as we should have.
1) We didn't destroy the Communist forces.
And,
2) We didn't keep our word to support South Vietnam, should the North invade after US forces left the country.
The South was able to fight off the NVA and was doing a good job, until they literally ran out of gas and ammo. Our promise to make sure that didn't happen wasn't kept. I believe, from the research I've done, that the ARVN could have been successful against the NVA, had we conducted massive airlifts of weapons, ammo, food and fuel.
Instead, we did nothing, while The North took down South Vietnam, killing one million South Vietnamese civilians. Pol Pot was freed up to takeover Cambodia, killing a million+ and there were over 100 thousand Hmong tribesmen killed in Laos by the Communist forces there. Remember the whole, "Yellow Rain", thing in the late 70's? Yeah, that was the Pathet Lao Communists using mustard gas to wipe out the Hmong.
To this day, the Hmong are still being murdered by the Communists in Laos and Vietnam.
Laos, Vietnam Troops Slaughter Lao, Hmong: Over 6500 Face Death | Soldier of Fortune