It isn’t as closely connected to our own history and culture (for most Americans).

The names and places are barely familiar if at all, and hard to pronounce. We don’t have huge numbers of refugees here in the United States from those massacres.

And you can’t overlook the fact that for Jews the Holocaust and certain Stalinist policies are just episodes in their long history of persecutions. This kind of thing is part of what it means to be Jewish. There’s an institutional memory behind it that probably isn’t there behind some of the things that happened in Asia.

Or if it is, there aren’t as many Americans with ancestry who experienced it. A huge proportion of east coast Jews have at least one relative who died in the Holocaust or fled Stalinist Russia.

Finally, languages and literature. A ton of very poignant and often read books and movies deal with the Holocaust and or Hitler in one way or another. From Hogans Heroes to The Sound of Music, Life is Beautiful, Diary of Anne Frank, The Hiding Place, Mans Search for Meaning, etc.—and tons more. All of those were translated into English a long time ago and have become classics here.

I have no idea if a similar literature exists about the Asian atrocities but if it does it doesn’t seem to be as widely available in English.