Because Russia actually did start a conventional (so far) war over potential military incursions into its sphere of influence.
Whataboutism itself is dismissive. It's justifying bad actions because other people have done bad actions too at some other point in time. If I murder a guy and get arrested, I don't get to go, "But what about Jack the Ripper? He murdered far more people than I did and he got away with it! You don't get to criticize and arrest me when Jack the Ripper was allowed to murder prostitutes for years!'"