The USSR indeed failed, but it wasn't an "experiment" in the sense of putting ideas into practice under laboratory conditions. Perhaps those who are hostile to historical materialism can't accept the idea that the revolution isn't an "experiment," so they can't understand failure either.
Left up to anarchists, the Russian revolution would not have happened, and the USSR wouldn't have come into existence. Perhaps anarchists think that it would have been better had the USSR never existed.
Yoshie