Yes, the excuse is that Marxism has never “really” been tried. Well, the reason is that it is a flawed philosophy that CANNOT work. It is inevitable that Marxists resort to dictatorship, force and gulags to try to impose “real” Marxism. How many millions have to die before the truth penetrates those leftist skulls?
Technically, the "Dictatorship of the State" is the approved 3rd Phase of Revolution in Communist dicta. It is understood that this phase is necessary in order to effectively "impose real Marxism".
But, somehow, they never seem to reach the 4th (and final) Phase of the Revolution: The Withering Away Of The State.
Perhaps, that's simply because the 3rd Phase has never been pressed to a successful conclusion...