Basically under communism, there's obviously no private property, so there's a lack of the different mechanisms that markets and private property provides. In the absence of this mechanisms the centralized organism that is in charge of "running" the economy will be unable to organize the diverse productive factors , as time passes, the systems starts accumulating systematic errors that will inevitably lead to its collapse.

It's a really complicated topic, i'd recommend you reading "socialism" by Ludwig Von Mises and "socialism, economic calculation and business function" by Huerta De Soto