According to an Educator, Scott M. Stolz, who analyzed the atrocities of Adolf Hitler, said one reason is the nature of the deaths attributed to Hitler, the other reason has to do with who wrote history.
Organized Genocide vs. Political Killings
Hitler is well known for organizing genocide on a mass scale, trying to completely eliminate entire groups of people, most notably the Jews, but also Gypsies and other targeted groups. Stalin and Mao are well known for mostly targeting people for political reasons, killing opponents and dissidents. While this is obviously a bad thing, genocide is considered worse.
Hitler Lost a War / Stalin & Mao Did Not
Another reason is that Hitler lost a war and was replaced by a government hostile to his regime, while Stalin and Mao did not lose a war, and were replaced by leaders who were part of the same regime. People from the same regime have a vested interest in downplaying anything that makes the regime look bad, while people from a new regime have a vested interest in making the previous regime look bad.
Killing Fellow Countrymen vs. Foreigners
Also, Hitler invaded other countries and killed their citizens, while Stalin and Mao killed mostly their own countrymen. This means Hitler personally affected people in many more countries than Stalin or Mao did during their reign.
The Winners Write History
The victors write history, and the winners in World War II made sure that the truth about Hitler came out. Since the Soviet Union was also a winner, they conveniently wrote history in their favor demonizing Hitler, while downplaying anything Stalin did.
Also, there is dispute over what should or should not be included in the numbers attributed to each leader. To properly compare the leaders, only killings related to genocide and political killings should be included if you are trying to compare how “evil” they are. Including people who died due to bad domestic policy (i.e. starved due to bad policies) or in war skew the numbers. If you include people who died in war, every leader has blood on their hands, unless you try to attribute the deaths to the losing side. But then again, that would be the winners writing history, not an objective look at what really happened.