Kent state happened so long ago that most American students just don't know, or anything much that happened during Vietnam, which our history classes don't seem to have time to cover. In china, its a bit worse, as they have the siege of Changchun (the communists basically starved out a few hundred thousand civilians), or the 1938 yellow river flood, where the nationalists killed millions of civilians to thwart the Japanese. In those contexts, Tiananmen is just a blip.