Because it intentionally neglects the harm caused by the Great Leap Forward, or various soviet famines. The statement is designed to deflect attention and responsibility from A to B and use hyperbole to create outrage.
Its possible to argue that both China and the Soviet Union were not actually communism but rather authoritarianism but for the sake of common understanding we will refer to them as they refer(ed) to themselves.
So did millions of people die because of "communism?" the facts say yes. So do millions of people die because of Capitalism? the facts say yes.
Both are true statements, and using hyperbole and outrage to deflect or minimize one or the other is in bad faith.