Specifically, whataboutism contradicts commonly understood logic. We all understand that when you make a claim you should be able to universalize that claim, situationality not withstanding. Put more plainly, we all recognize that if you make a claim like "Good people are honest" you should be able to apply that judgements to any context for it to be a true statement, with the exceptions of course being the occasional situational ethics. If you make a claim and can't apply that generally to other situations then your claim is wrong because it fails to be an accurate generalization. For example if I tell you that "Good people are honest" and "You lack honesty, therefore you aren't a good person" you might respond to that by saying I also lack honesty. You're not deflecting when you do that, you're simply pointing out that I can't unversalize my claim. I either have to be arguing that I too am a bad person, or the claim I'm making can't be a good generalization and therefore can be discarded.
Now let's apply that to politics. If I claim "Countries that treat LGBT people poorly are bad; the USSR historically treated LGBT people poorly meaning the USSR is a bad country" you might respond to that by saying "The US also treated LGBT people poorly in that time period; also the USSR treated LGBT better, sooner than US". You're not deflecting when you make this claim. Like above, you're arguing that my statement "the USSR is bad because it treated queer people badly in the 50s" would disqualify my own implicate position that the US is good. You're pointing out that I'm being manipulative by holding the USSR to a standard to which I don't hold the US. You're pointing out that I clearly don't actually want to use a state's treatment of LGBT people as a yardstick for its morality because I'm not holding my position to this general standard - much in the same way that you might tell me I don't care about honesty in the first example.
Yet liberals and other reactionaries immediately go "oh that's whataboutism" and move on, ironically ignoring the fact that if they were to universalize this point nearly every person would be guilty of invalid argument on a frequent basis. It's a contradiction to the ways we intuitively understand arguing and ethics. Whataboutism as it's often used is not a fallacy or a bad argument tactic, it's that liberals and other anti-communists fail to recognize that they have implicitly stated beliefs which they are contradicting when they argue against socialist states. They've been trained to see their value statements being questioned as an invalid tactic, specifically because if they were forced to examine the contradictions in their value statements they might become socialists
Making it into a pseudointellectual argument (i.e. "you're using a fallacy") also really helps liberals discredit it. Ideologically, American liberalism styles itself as a technocracy; the political class and those who support them are simply more understanding about these issues than the common person. It's why you'll see libs parrot MSNBC talking points about why we can't do this or that radical thing. They see themselves as more qualified to be making decisions than others and are therefore susceptible to arguments which stroke that ego about being one of the smart ones. By making the counter "you're using a logical fallacy" libs can say to themselves "oh its one of the uneducated, since they can't argue properly. I don't need to listen to them or question if this reaction is appropriate because I'm so much smarter and more politically knowledgeable than them"
Yeah good point. It’s the same reason why people have positive views of “meritocracies” and believe the lies about capitalism being based on merit and “hard work”.