How do you know that? I'm genuinely curious
Pre-WW2, you've got millions exterminated in the Congo Free State. The Ottoman Empire killed a million Armenians, 500,000 Greeks, 400,000 Assyrians. The British starved five million Indians to death in 1900 and two million Bengalis in 1943 and nobody gave a single solitary shit.
Post WW2 it gets worse. Two million people in Cambodia, 700,000 people in Indonesia, 550,000 in Rwanda, 500,000 in Angola, millions in China during the Cultural Revolution, 225,000 in Bosnia, 300,000 in Uganda, a million Bangladeshis in 1971.
The list goes on and on and on and the fraction that prompted any kind of forceful international response is pathetically small.
Congo - when the atrocities came out, the king was forced to cede it to Belgium and the terror more or less stopped.
The Ottoman genocides were during the Great War, and even their allies ( Germany ) were concerned and issued proclamations, let alone their enemies ( Russia) which used it for recruitment.
Cambodia, Rwanda, Bosnia ended with wars/peacekeepers to topple the genocidal governments.