This is an extremely difficult question, but one of the reasons was obviously the Holocaust and the systematic attempt to exterminate the Jewish people and many other "undesireables." But the difficult part of the question is why this was emphasized over (and continued to be) other despots who've killed as many or more people than Hitler did, Stalin for one.
Some would argue that Hitler was portrayed as so evil because his country lost the war whereas Stalin was helping us win so we were not as quick to villify him. He also was not as public nor in some ways systematic about his mass murders, though it is likely that he killed a far larger number of people within the Soviet Union.
I tend to think that it is partly the incredibly systematic nature of the Holocaust and the incredibly organized way it was carried out that makes it so uniquely terrifying.