Follow up question, why were the Soviet Union and Vietnam so opposed to a supposedly fellow Communist nation?
https://en.wikipedia.org/wiki/Khmer_Rouge#Place_in_the_United_Nations
Why did the west push so hard to support a genocidal government over a non-genocidal government?
Because they were a proxy of Dengist China, a US ally and antagonist of the USSR and Vietnam.
Follow up question, why were the Soviet Union and Vietnam so opposed to a supposedly fellow Communist nation?
The Khmer Rouge were not the preeminent Communist Party in SE Asia, it was the Indochinese Communist Party which was a mass non nationalist based Party. This organization eventually split off into the Worker's Party of Vietnam and the Khmer People's Revolutionary Party, which Pol Pot was a high ranking member of but abandoned due to it's ties to the Vietnamese.
Up until the late 60s the North Vietnamese supported Pol Pot's group.
The Khmer Rouge were highly secretive and did not call themselves Communist until 1966 after Pol Pot visited China and was pressured to change their name to the Communist Party of Kampuchea and attempt to wrest control of the Communist movement from the Vietnamese. It wasn't until 1977 that they publicly changed their name from The Organization to the Communist Party.
Between the late 60s and the late 70s the Khmer Rouge began to purge Vietnamese and Vietnamese allied Cambodians from the Party apparatus and the State itself, began to engage Vietnam in border clashes, etc.
Here's a post I made else where that details the historical problems between the Cambodians and Vietnamese that had a hand in why the Khmer were very antagonistic towards them.