Not a contributor, just an armchair historian but I hope this answers part of the question. Donald W. Beachler wrote in "Arguing about Cambodia: Genocide and Political Interest" Holocaust and Genocide Studies, Vol. 23, No. 2, Fall 2009, p. 214, 215:

"Many of those who had been opponents of U.S. military actions in Vietnam and Cambodia feared that the tales of murder and deprivation under the Khmer Rouge regime would validate the claims of those who had supported U.S. government actions aimed at halting the spread of communism. Conservatives pointed to the actions of the Khmer Rouge as proof of the inherent evils of communism and evidence that the U.S. had been right to fight its long war against communists in Southeast Asia..."

Basically, some left-wing academics such as Noam Chomsky were hesitant to believe the claims being put forward about what Pol Pot and the Khmer Rouge were doing, due to sometimes the nature of the evidence itself, but also because of their own biases. They hoped the Khmer Rouge would actually be a Socialist success story and affirm their beliefs about the flaws of American foreign policy in Southeast Asia at that time. Keep in mind that these scholars were not allowed to visit Cambodia and many if not all foreign journalists were expelled from the country early on. These scholars had also not spoken to any of the refugess or few foreigners living in Cambodia during that time. This led to what Sophal Ear called the "Standard Total Academic View on Cambodia" (STAV). To be fair to Chomsky, he did eventually start referring to the genocide as fact by the 1990s, and many other prominent genocide skeptics recanted their criticisms.

Please if anyone else more experienced in this subject wants to answer, do so.

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