Very good point.
Stalin did his best to hide his crimes, there were very few films. Case in point: Stalin had the Ukrainian borders sealed in 1931 and starved to death between 5 to 10 million peasants. A plus or minus 5 million people is a lot, but no one knows even which million numbers of deaths occured. They starved to death by the untold millions, their names are unrecorded, and even photographs are rare. Ukrainians to this day are still trying to find records of their Holocaust.
The same for the tens of millions Mao had killed, they weren't filmed or photographed, they just 'disappeared' and exact numbers (to the tens of millions) aren't known.
I guess part of the attention on the Nazis is that they were such good book keepers and filmmakers. You could see mass-murderers at work.