First of all I'd like to say historical atrocities are not a contest. In the end, it doesn't really matter who killed more people, what matters is realizing that all historical atrocities are horrible, and that we should try as hard as we can to not repeat the mistakes of the past.

Second of all, the reason why the holocaust sometimes (certainly not always, however) gets more attention than the crimes of Stalin or Mao is because of the unprecedented nature of the holocaust itself. Never before had a nation constructed an entire industrial system that existed solely for the purpose of wiping a particular group of people off the map. Thats what makes some people look at the holocaust as a singularly terrible atrocity.

The Holocaust looms large because it was an intentional effort. Mao and Stalin reorganized their economies in such a way that caused starvation and death. They also wielded violence to bolster their power. But the Holocaust was unique in that causing death was its very goal. Furthermore, the tallies for Stalin and Mao are comprised mostly of people who died inside of work camps (unintentionally) or during famines, so it's much more problematic to say that Mao/Stalin killed X # of people than to say Hitler killed X # of people.

The reason that the Holocaust is 'different' is the percentage of the worldwide Jewish population that was targeted and killed.

The European Jewish population in 1933 was about 9.5mil. 60% of the world's Jews lived in Europe at the time.

This means that about 1/3rd of the Jews in the world were killed in the Holocaust.

You've gotten some great responses, but in addition to that remember it's wasn't "just" 5.6 million jews that were killed. Add in another 5 million + that were killed for a variety of other reasons including being gay, gypsies, soviets, communists, and even being mentally challenged. So you're looking at a death toll of about 11 million people. Considering the European German population at the time was around 80 million that is a huge chunk of population being slaughtered intentionally. As others have pointed out Stalin and Mao Zedong weren't going out to slaughter huge portions of their population. Most of those deaths happened as a result of other actions that were taken (labor camps or famine for instance). They weren't purposely annihilating 1/8th of their population.