Published at 07:57 on 27 March 2022
Basically, because although NATO is on paper a multilateral organization, the USA is clearly and by far its most powerful and influential member, and therefore its de facto leader. Thus, any remarks of such character by a U.S. president help make Russia’s case that this whole thing is about a confrontation between NATO and Russia.
Since NATO is clearly more powerful than Russia and its allies, that, in turn, helps Putin reframe the narrative from one in which Russia is the aggressor to one in which Russia is being threatened by a yet more powerful alliance bent on ganging up against it. And the more Putin can do that, the more secure his position becomes.
Biden said what is obviously true: both Russia and the world as a whole would be better off without a fascist like Putin in charge. But by the very fact of saying it, he made that goal more difficult to achieve.
To reiterate: with great power comes great responsibility.