in theory communism makes resources the property of everyone. In practice, somebody has to manage the resources, and in doing so controls them. if you control the resources, and especially if you have the power to defend that control, you effectively own them. this means that while in theory the people own everything, the truth is that the government owns everything. by similar logic to the first bit, if they own everything then they have control of everything, meaning that they are authoritarian. that being one of the major defining characteristics of a dictator its not far from there to become one even if you aren't trying to. ironically, this is all how it's intended to work save for the dictator part. this is the apple mouse charging port of communism, an intentional thing with a bad result that can't be given up without invalidateing things you have said or done.
also, communism tends to be brought about by coups and coups tend to lead to authoritarian leaders, compounding the above issue.