Because they’re still competing with us. A communist world doesn’t matter to them now, but a weaker US vs-a-vs them does. They know communism\socialism kills economies better us then them.
I don’t know about it not mattering to them now. They do still back the Nicaraguans and Angolans, they do still back China, they do still have that Karl Marx statue and Lenin’s Mausoleum up and running instead of, say, torching them or tearing them down, and making sure what’s left of Lenin’s corpse is hung from a street lamp, and Putin strongly implies that he himself still adheres to Communist views.
And personally, I’d prefer a Russia that fully gives up on Communism to the extent of utterly destroying any trace of it than one that pretends to have given up on it yet when looking closely at it still worships it to this day.