The war with Vietnam was widely unpopular at home, especially after newscaster Walter Cronkite, a highly trusted figure, came out against it in early 1968. Frustration with the war and the draft had led to antiwar activism, causing domestic unrest in the United States. Further, by the late 1960s, the US had determined that the war was not winnable. Richard Nixon won the 1968 presidential election in part by promising to end the war quickly and with honor.
Nixon had high hopes that the US could negotiate a peace settlement by the end of 1969, but these plans came to nothing. The Viet Cong and the US simply found it impossible to get to a place in the negotiations where they both were comfortable, so the war went on through the early 1970s. The US spent this period withdrawing US troops and engaging in what it called "Vietnamization," which was a plan to equip the Vietnamese to continue the war themselves with the backing of US military supplies and air power.
Nixon, however, became preoccupied with Watergate scandal and finally resigned. While President Ford was in office, in 1975, the Viet Cong initiated another massive assault. At this point, the United States was worn out from the protracted, pointless, and very unpopular struggle and made the choice to withdraw. At the end of April, 1975, the Viet Cong took over Saigon, the South Vietnamese capitol, ending the war.
In retrospect, it is clear that the Viet Cong, though vastly under-armed and under-equipped in the fight, had more "skin the the game" that the United States. For the Viet Cong, this was their homeland and their terrain. They had no reason not to keep on fighting. They knew they could rely on support from both China and the USSR. The US was fighting in a tiny country halfway across the planet and simply didn't have the same investment in winning. Finally, some argue that though the US lost the battle for Vietnam, it won a larger "war" by keeping South Asia from becoming communist.