Because the goals were different. Stalin's atrocities were about political repression and expediency. Hitler's were ultimately genocidal. When examined on the small scale it's the same reason people react differently when hearing that a murderer killed someone in a mugging as opposed to in a hate crime. I'm not trying to make a direct analogy between the crimes, just trying to point out that the motivation that drives a crime is often a major element in how people perceive that crime.