There were a number of factors that contributed to the Khmer Rouge taking power in Cambodia, and they were affecting all of Southeast Asia, not just that country.
Remember that the Khmer Rouge took power through armed aggression, a rebellion against the government, as opposed to widespread popular support or a democratic election. But their rebellion became more widespread and successful after a lengthy, multi-billion dollar bombing campaign by the United States failed to change the course of the war and served as an excellent recruiting tool for the rebellion.
In April of 1975, South Vietnam fell in only one month to the North Vietnamese Army, and Laos fell as well, to the Pathet Lao, with Cambodia being taken over by the Khmer Rouge. Eisenhower's "Domino Effect" prediction became reality.