If you'd like to know our reasons for doing that (or anyway, my reasons), they're twofold. First, even with the rise of China and the seeming return of global multipolarity, western countries continue to dominate world affairs, militarily and economically. It's simply a matter of scale; when they act up, it's likely to effect the lives of millions of people around the world, directly or otherwise. Second, most of us are ourselves westerners, and of those I'm betting most of us are from the the US. As citizens of these places, our first responsibility is to point out our own nations' crimes, both because of their widespread influence, and because of our proximity to them.

There's also the matter of communist countries being the subject of a truly absurd amount of western propaganda. We feel the need to push back against certain narratives about the supposed crimes of communist countries because we know many of them to be exaggerated, misrepresented, and at times outright fabricated. Most of us are close students of history, some of us like myself are even academic historians, and it can be frustrating to provide reams of evidence for our claims (or more often, counter-claims) and be met with accusations of whataboutism, rather than earnest engagement. It's why so many of us are quick to assume that the pushback we get is in bad faith, because it quite often is.

But anyway, I'm getting off track. Very few of us, I find, are unwilling to acknowledge the flaws, missteps, and yes, even crimes of actually existing socialist states, when they are well evidenced. For instance, I doubt many of us would defend the deportation of the Crimean Tatars, but we're equally unwilling to accept the Holodomor as an example of deliberate ethnic genocide because the common narratives surrounding it rely on fabricated numbers, misrepresentations of Soviet state policy, and Nazi propaganda, to say nothing of their denial of professional historical consensus.