The United States prevented an election in South Vietnam because:

The U.S. didn't sign because they were not willing to accept reunification under communism, Diem refused free elections because he knew HoChiMihn would win.  They told the South Vietnamese military they would not object to his overthrow so military leaders staged a coup, seized the government, and assassinated Diem.

a drastic rise in oil prices ended decades of postwar.