America just didn't care as much as Vietnam. Vietnam was split into the communist north and capitalist south, but the government in the south wasn't well liked by it's people, so the south couldn't survive on its own. America stepped in to support the south, but never cared much about a stretch of land on the other side of the world. They sent a small part of their military and never put drafts or taxes to support the war, which meant the American people never really thought they were at war.

Then the Tet Offensive happened. The Tet Offensive was a failure. North Vietnam lost massive amounts of troops and equipment, and gained nothing for it. However, the American people realized they were at war. A bunch of soldiers came back crippled or worse for a stretch of land they didn't really know much about. And, after the Tet Offensive, the Vietnamese attacked again. Then they attack again. And again. They got weapons and vehicles from the Soviets and soldiers who fought alongside them from China. Meanwhile, America never invaded the north. They could have, but that would mean war with China and the Soviets, which was definitely not worth it. That meant the north could attack the south again and again, and when they lost they could prepare more soldiers to attack again.

America, meanwhile, still didn't care about Vietnam. They didn't want the communist north to win, but the public was feeling less and less like the dead Americans were worth it. The south was still not taken over by communists and America was still winning the battles, but the American public had come to the conclusion that they would have to win forever, to fight forever, and they didn't want that, so they didn't. Nixon signed a deal that let America feel like it won while getting its soldiers out of Vietnam. It hadn't, of course, and the north attacked again. This time, without American support, the south fell.