I had wondered about this weeks ago in history class, as we read about Imperialism in Asia and Africa. Throughout the chapter we read about things like the Sepoy Mutiny and the Opium Wars, but one paragraph stood out to me. It was about the Taiping rebellion, an event that caused over 20 million deaths. How much did the chapter divulge on it? Less than a paragraph. Why is an event with more deaths than the holocaust so easily looked over?
I apologize for commenting without the appropriate sources but the primary reason that the Taiping Rebellion is covered less than other conflicts like the Sepoy Mutiny and the Opium Wars is the Euro-centric focus of history classes in the West. The Sepoy Mutiny and the Opium Wars both involved a significant change in the British role in world affairs while the Taiping Rebellion was mainly a Chinese civil war although both the French and English aided the Qing government in putting down the rebellion. As this is simply from what I was taught in college I feel it'd be best to delete this comment once a more qualified answer is posted.
Similar to how the American civil war is not taught in history text books here in India, whereas the American war of independence is taught depending on the curriculum chosen by the school. The former had few significant diplomatic ramifications (apart from the U.S. being unable to intervene against Napoleon III's invasion of Mexico), whereas the latter had a major impact in upsetting the British balance of power in the new world.
There are other events in Chinese history that are rarely discussed too. For example, the An Lushan revolt ended in the deaths of 1/6th of the world's population (the figures are controversial, however). Then there are the Dungan and Panthay rebellions, which resulted in the genocide of Hui Muslims and Miao hill people. Despite the high casualty rates, these are events in Chinese history, and are of little significance internationally.