There was no political appetite for intervention against the Khmer Rouge. The US had just left Vietnam and there was no desire for them to re-enter South East Asia (in fact there was a strong push for them to stay out from the Left, see Chomsky's articles on this). The Khmer Rouge kicked out all of the western journalists, so the only information coming out was from refugees fleeing into Vietnam.

Also humanitarian intervention was very rare between 1945 and 1989. It was generally held to be illegal under international law (after the Khmer Rouge were overthrown, both France and Norway explicitly said that their crimes did not justify an intervention) and due to the Cold War it was much more risky (Kampuchea was aligned with China, so any US or Soviet intervention could have provoked a response). It wasn't like today when it's generally accepted that we can invade countries in order to prevent genocide.

Of course eventually something was "done", Vietnam invaded and replaced them. They didn't do it sooner as they had no reason to and believed they could have good relations with the Khmers. They only did it in the end because the Khmers began crossing the border to kill Vietnamese. As I said, they were condemned in the UN for it (they actually claimed that they had not intervened and that the Rouge was overthrown by the Vietnamese people, and they accused the Chinese of being the real interventionists for propping up the regime).