Could you be more specific and quantify what you mean by ‘swept under the rug’? In much of the West (certainly the English and French speaking parts), which is the perspective I assume you mean, the Khmer Rouge, Pol Pot, and Cambodian Genocide are indeed well known, and often taught as one of the most severely brutal, rapid and proportionally devastating genocides there has been, from school textbooks to a film that won three Oscars. Should it be more widely known and discussed, and is there a lot to say about its historiography in the West? Of course. Was it taught in every school curriculum? Probably not, but very few things are taught everywhere - the reasons might be rather specific to which curriculum you’re referring to.
But it’s rather unusual to expect it to be as well known as the Holocaust when:
- The Holocaust (specifying Jewish victims) killed nearly 6 million people, and by extension to the systematic mass murder of others by the Nazi regime perhaps 11 million, and millions more including all civilians killed by the regime - while the Khmer Rouge killed 1-2.5 million people.
- The Holocaust happened in European countries whose culture and languages are more accessible to the West, and moreover across a huge swathe of Europe, affecting what are now dozens of countries directly. What gets most famous isn’t decided by committee but links such as these - events in Cambodia are similarly more prominent in Cambodian cultural memory.
- The aftermath of the Holocaust was encountered by Allied countries who marched across Europe and liberated the camps - the bulk of the death camps mostly by Soviets, but also Americans liberating the likes of Dachau and Brits the likes of Bergen-Belsen, each leaving a lasting memory for large numbers of troops in those respective countries too. This has also led to far more details being uncovered - not only did the Nazis keep meticulous records, but a huge proportion of them fell into Allied hands - part of the reason why we have a much better idea of the figures above. Instead, the Khmer Rouge were ousted by Vietnam, which occupied Cambodia for 13 years, with authority held to be illegitimate by the West, before power is as handed over to a brief UN-run transitional government.
- The Holocaust was perpetrated by the regime that was the enemy of countries in the deadliest war in all of human history, placing it at the heart of modern Western history on those grounds alone. Bluntly, it was a bigger genocide part of a much bigger war. The long, dark history of anti-Semitism throughout the West has also led to focus on it as an educational guard against both that and intolerance in general, where an equivalent for Cambodia in the West is not quite as clear, especially considering the earlier and numerically greater deaths under Stalin, and for that matter Mao.
- Some 3.5 million European Jews survived the Holocaust, and millions of Jews who never knew Nazi rule, and they and their relatives have had far greater cultural connection to and disproportionate cultural influence within the West (in a neutral/positive sense) since then, with over a generation longer to do so, from books to films and direct contact with survivors.
- The Holocaust has had a generation longer to be discussed and investigated, with the word genocide being coined in its wake; when the UN, early organisations of European unity, the modern state of Israel, and several prominent human rights organisations were founded, the Holocaust was in fact held up as the example of genocide, hardly unexpected given the facts and the above.