Short answer is because Sihanouk was a utility. The Khmer Rouge, despite their ‘anti-feudalist’ rhetoric, heavily leaned on him to increase recruits within a rural population where many still regarded him as quasi-divine. He was also well connected internationally with sympathetic governments including China and North Korea. He could act as a figure head for the regime before the CPK and Pol Pot ‘came out’ as the leaders of the new state.

Many members of the royal family were killed however. For instance Sirik Matak, one of Sihanouk’s cousins (there were a lot of cousins as kings had many children) was an instrumental figure in organising the coup with removed him from power in 1970. He was also one of the first members of this government to be murdered once the Khmer Rouge took Phnom Penh.

Sihanouk said that many members of his family died during the period from 1975-1979. While he remained in relatively pampered conditions during the regime (he bemoaned not having enough bananas for his flambé) he was still deprived of many freedoms. He said that he expected to be spat out like the pit of a cherry once the CPK leaders were finished using him.

Which is an interesting question - would Sihanouk and other prominent members (the queen mother/his wife) have been killed if the Khmer Rouge had remained in power for a longer period of time? I do not know the answer to that, but my original assertion that he was not killed because of his utility, is reinforced by the events following the demise of the regime in 1979. Once again the leaders needed Sihanouk to act as a legitimate ruler and plead on behalf of the Khmer Rouge for assistance in defeating the ‘invasion’ of the Vietnamese.