It's very difficult to explain why people don't know things, or even to be sure that they don't. If the premise of the question is true, then likely answers are:

1. The Indonesian killings took place 10 years earlier than those in Cambodia. ten years is a long time in the public memory of current affairs.
2. The victims of the Indonesian killings were communists (and their associates). Even if Western powers did not connive directly in the killing, they were not upset to see the Indonesian Communist Party destroyed. By contrast, the Khmer Rouge were communists and widely seen as enemies of the west.
3. The Khmer Rouge regime was overthrown in 1979 ands the successor government has a strong interest in documenting KR crimes. By contrast, the Suharto regime remained in power in Indonesia for decades and, at very least, discouraged close investigation of the killings that had helped it come to power.