Why is “Whataboutism” often criticized?

Whataboutism or Whataboutary is the technique or practice of responding to an accusation or difficult question by making a counter-accusation or raising a different issue.

Examples:

The USA to Russia: "You invaded Ukraine and killed tens of thousands of people."
Russia to the USA: "What about Vietnam, Iraq, Afghanistan,...?"

Now, my question is, why is "Whataboutism" often criticized?

Those who criticize whataboutism, What better type of reply do they suggest?