Apart form PR there's also another good reason for not including Hitler: He wasn't that great a leader actually.
Hitler essentially completely ruined his country, left it divided, occupied and resulted in Germans being driven from lands which they had been living in for the past centuries. Sure he created a short period of economic revitalisation (which was not sustainable though) and conquered a large area which he held for about three years. But in the end he completely ruined the country he took over.
There are simply dozens of better leader choices for the German civilization.
Well he make a weak, economically crushed Germany strong again. Without WW2 and the holocaust probably he would be considered the greatest german leader of 20th century.
Stalin is the same story. He killed millions but made Russia superpower.
But I don't think Mao should be in the game. While he was a great milliary leader he was also a terrible political leader with the idea of Great Leap Forward or the Cultural Revolution. Deng Xiaoping was the modern leader who made the China the country we know today.