“They didn’t want to” is hiding some of the complexity behind it. Vietnam had turned into a political crisis at home; there were widespread protests and lots of people openly dodging the draft. Grunts would sometimes “frag” commanders who pushed them into overly risky missions. Running up the casualties would have created a real risk of a violent revolt at home and mutiny among the soldiers. The US as a society was likely incapable of winning the war, so it turned into a defeat.

There may have been a window earlier on when greater commitment could have won the war or frozen the conflict, via invasion of North Vietnam, but not once the public turned against it. But this came with a big risk of direct intervention from China or the Soviet Union, as happened in Korea. In Korea, Chinese intervention led to a stalemate.