Why would the US want to leave Vietnam rather than securing total victory?

By the late 1960s, US political and military leaders got convinced they could not win the war by any means. The war would extend indefinitely - an unacceptable option - and the Communist North Vietnamese were determined to resist as long as it was necessary, despite the horrendous suffering and destruction; actually this is what the Vietnamese have done throughout their history against foreign occupiers since ancient times. The exorbitant financial cost of the war was also a major headache for the government. And there were the rising numbers of American casualties, a fact that was politically negative. Another major factor was the powerful peace movement in the USA, many people were opposed to the continuation of the war, social support for US involvement in Southeast Asia wasn't strong. The US could not win that war.