Today I learned of the atrocities committed by Mao Zedong for the first time. I had heard the name a couple times before but had no idea who he was or what was significant about him. I polled a dozen of my friends (seniors in high school and college students) and very few of them really knew anything about him as well.
Why are Hitler and Stalin such huge names for their atrocities and this man isn't? Is it just because they were involved in World War Two and he wasn't? It just seems strange to me that so few people seem to know about a man who is believed to have killed more than both of the them combined.
I think one huge difference is their intent. Intent matters significantly in a murder trial and it applies here as well.
For Hitler, he is vilified because his intent was to commit genocide, to explicitly remove vast populations of Slavs and Jews. He went forward and carried it out, murdering millions for the sake of murdering millions.
For Mao, the majority of millions of casualties under him are due to failed economic and social policies. The intent of these policies, as you will see, were to benefit the country and populace. However, due to lack of expertise and knowledge, things went wrong.
Let me give you two examples which contributed to the cause of the great famine in 1959-1961 that killed millions. In 1959, Mao started a campaign to remove/kill "four pests": rats, flies, mosquitoes and sparrows. Sparrow was considered a pest for their appetite on consuming grains. The intent was in itself good, the goal is to remove agents that spread disease and destroy agriculture. However, what Mao didn't understand is the interdependence of all species in an environment. By removing sparrows, which also hunts insects and locusts, other pest populations boomed, which had even more devastating results.
The second example is more well known, which is the great leap forward campaign. The intent of the campaign is to industrialize the country and improve its manufacturing capabilities. This vision isn't bad, but the way it was carried out was terrible. People abandoned agriculture and produced inferior steel products to meet "quotas".
Now Mao did a lot of other terrible stuff, but a bulk of the millions that died under him were from these failed social policies which had good intent. So a direct comparison between Hitler and Mao would be disingenuous.
I think you are correct with the involvement of Hitler and Stalin in WWII as a large factor in this. Most people (who aren't historians or students of history) have at best an unsubstantial grasp of U.S. history. Most high school curricula doesn't stray from U.S. history and offer other focuses as electives. Also the U.S. did not recognize the PRC until 1979 (instead recognizing the RoC or Taiwan) which could also explain the lack of focus or mentions within the scope of U.S. history.
In the United States Mao simply is not as large of a figure. American culture is self-serving, we focus on our own involvements, and Mao was largely not associated with the West. For that reason you are just not going to learn about him as much. Mao was portrayed in various art works by Andy Warhol when he was contemporary, but current cultural focus in the United States simply doesn't care about eastern affairs that do not affect the country. Think about how many Chinese books you know or other prominent Chinese historical figures.