I'm thinking the German genocide stands out because of its direct link to the ideology of the party in power. I believe other genocides were ultimately, well, more "practical" matters.

Some of it is undeniably because the Germans were the enemy of "#1". The U.S. also seems to adopt ideologies only in times of war (which is followed by relative ideological vacuum), so the Germans of that period have the honor of being colored to some extent, still to this day, in the light of the 1940's U.S. mindset.