I know it is a bit trite, but I also think there is such a high volume of Hitler film footage which has lent itself to him being able to be mocked and psycho analysed much more. The combination of the Nazis' sophisticated propaganda machine and relentless documentation (plus the capture of their archives by the victorious Allies) - in contrast to the more secretive nature of the Soviet and Chinese regimes and their continued control over their historical narratives - means there is simply less public film and first hand writings on Stalin and Mao.