If you mean fascist dictatorships and authoritarian capitalist states disguised as communism, you have the tankies to thankie for these not being condemned on sight amongst communists. If you mean Marxism in general, then it's because it is in fact a good thing in principle and the polar opposite of nazism.
The problem isn't in the theory, it's in the application: state communism invariably leads to dictatorships and crimes against humanity, even if the intentions are to "protect the communist state" and keep it from falling into capitalism again. That's not an excuse, however, which is the point of contention between tankies and other communists, as in, that the end does not justify the means.
In the end, the only kind of communism that can hopefully work without becoming a dictatorship is the anarchist, communitarian type. Either that, or electing a communist party and keeping the same political and economic system, but then that won't be communism, it'll be leftism, socialist democracy, what-have-you, and any deep social and economic changes that try to challenge or dismantle capitalism will be resisted by banks, corporations, and political institutions that would never allow such measures and have the power and capital to prevent them.