Because most historians can uncontroversially (within their field) say that the Holocaust was an intentional genocide. There's really no debate behind Hitler killing people, most of them completely innocent civilians, because he thought them to be subhuman. This extends to the general populace as well: most people agree on this. And most people, historians or not, would say that's pretty "evil".

Stalin and Mao, on the other hand, is a lot more complicated. Majority of the numbers that surround them are a result of cold war propaganda and can't be taken unconditionally. The most often quoted source, for example, is The Black Book of Communism, which gives a lot of wild numbers with nothing to back it up. The book pretty much counts all people who died while Stalin/Mao were in power as their "victims", and then likely inflates it even more.

The main problem is that the vast majority of victims attributed to them are for things like famine and mismanagement. And I think the difference between negligence and killing people for being the wrong ethnicity, religion, race or political party is pretty self-explanatory.

Of course, Stalin and Mao did kill and imprison their share of political enemies, but the scale and means are really not comparable to Hitler's crimes. The extent and just how bad exactly it was very much depends on who you ask, while pretty much everyone can agree that Hitler was "evil".