Especially because the original criticism is usually a stand-in for an overarching moral condemnation. "Whataboutism" was a US defense for the fact that it was an apartheid state while criticizing political and financial liberties in the Soviet Union.

It also worked. The shame built up in the US so quickly that laws started being struck from the books. The US entered WWII with a segregated military, and by the time it marched into Korea, it was rapidly desegregating. The Soviet Union had successfully made its case that the US had no moral high ground, although it had not made its case for its specific set of limitations of financial and political liberties.

"Whataboutism" is just ad hominem for dummies. But if the real argument is about who is the better man, or what is the better system, ad hominem isn't a logical fallacy. It's simply a change of subject.