Communists have. Anarchists have. I know you're using the question rhetorically, it deserves an answer. Some say that it lies in praxis - the method in which lower-stage Communism is established; some say that it is because of Lenin's praxis, that is, Marxism-Leninism, not Marxism or Communism in general; some say that it is because the state must immediately cease during revolution (this is the position of the anarchists); some say that it is because of poorly maintained democratic models and incorrect transfers of power; some say that it is because of the action of counter-revolutionaries within the party.
It could also be what you said. But I do not think it is nearly as simple as you are making it; arguments have been advanced for the reasons I have mentioned. Where are yours?