It is my impression that the Holocaust made an enormous impression on Western moral thought, particularly as concerns the modern conception of human rights.

For example, the prosecution of Holocaust perpetrators at Nuremberg saw the first application of the idea of "crimes against humanity." Furthermore, the drafting of the 1948 Universal Declaration of Human Rights, an extremely important document in present-day discussions of human rights, was largely motivated by a desire to prevent a reoccurrence of the horrors of the Holocaust.

Also, before the Holocaust eugenics was a very common and well-supported philosophy in America. Many Americans even admired the Nazis for their "progressive" racial/genetic programs during the 1930s. The horrors of the Holocaust removed eugenics (and, to a certain extent, racism) from the realm of moral acceptability in America.

Basically, the Holocaust is remembered not only for its death toll, but for the impact it made on Western (and in particular American) moral thought.

EDIT: clarity

In addition to the points already made, we should add that the Nazis, the Soviets, the Americans and the British took an enormous number of pictures and shot miles of film of the event.

Some of the film showing the liberation of Dachau and Bergen-Belsen is just unbelievable - you can describe something, but to see it on film is a different thing. All those emaciated corpses piled high in boxcars or being bulldozed into mass graves ... the emotional impact is intense.

Let's not forget the Nazi mania for recordkeeping, so the bureaucratic, everyone-is-in-on-it, just processing paperwork aspect of the whole thing is very chilling as well as incriminating.

The other genocides don't have documentation and images the Holocaust has. Never mind the aesthetics of the matter - the evil looking Nazi SS uniforms, the "Work Shall Make Free" slogan ironically over the gate of Auschwitz, the process of selections, the entranceway paved with Jewish headstones, the orchestras playing when the new inmates (death factory raw material) arrived. Evil as art is another topic, but it is one more reason why the Holocaust is remembered.

Source: Explaining Hitler: The Search for the Origins of His Evil, Ron Rosenbaum.

We actually discussed this in my Holocaust History&Memory course today. One point my professor touched on that no one else has is that the US has the second highest amount of Jewish citizens after Israel. So of course these Jewish citizens would want to remember those who were murdered during the Holocaust. Lastly the reason we have the biggest Holocaust memorial is because for the US the Holocaust doesn't effect our national identity and divide our populace like it would have with France or Germany following World War II.

As far as the Khmer Rouge goes, America is complicit in those crimes. Our inhuman bombing campaign in Indochina was a direct factor that allowed for the rise of the KR. Here in America, we are very reluctant to tell our students about facts that contradict our "good guy" mythology.