This is something that I still find it hard to swallow.
Every time a communist government, or some a communist-fueled revolution was made, it was made with a lot of passion in mind - Lenin was very passionate in his beliefs against the Tsar, Che Guevara against the Cuban government, Mao to reshape China entirely.
It is as though every communist in their heart were very keen and zealous about their desire to overthrow capitalism, sometimes with an iron fist, almost similar to that of a zeallous fascist in the 1930s and 1940s.
And for this, there was a lot of fear and discrimination against communists - the Shanghai Massacre, the Red Terror, the Red Scare, the Reichstag fire.
For 40 years during the Cold War, it was as though every communist was unbelievably confident in their goal to implement a communist government in every country, possibly as passionate as the colour red as it is defined as the colour of communism.
And yet, with every attempt to implement a communist government, there were major political and economic problems - economic stagnancy, famine, poverty, corruption, political censorship and even use of political force and assault.
Why was this the case? How come the attempt to implement a communist government was often done with a passionate attempt, sometimes with attempts that end up in conflict or violent debates, ended up with a dictatorship that ironically had all the power and also made the nations very stagnant politically and economically?
This is my first response on here, so sorry if I'm not providing enough source or detail.
I would go a different direction than the other responses, arguing that it's not really about Communism, but rather about the inherent instability of revolutions themselves. A revolution, especially one as drastic as the communist revolutions that have occurred, literally rewrites everything on how governance is done. You have to answer every question from "How do we make legislation" to "How do police arrest people". To be able to make such sweeping changes, so quickly, you have to hold a large amount of hard and soft power, both to be able to fully root out counterrevolutionairy action, as well as to just be able to make changes.
There isn't a police precinct already willing to listen to the orders of a new government, the new government has to make that new police precinct from the ground up. And this is repeated with every single government function that needs to be changed in the eyes of the revolutionaries. This is even ignoring the fact that there are likely various factions within that revolutionary group that have differing ideas on how to implement all these new ideas. To make all these changes happen in such a way that is not dictatorial, it takes time, effort, and most of all, compromise and bureaucracy.
All this bureaucracy that needs to be done is not efficient, nor is it instantly successful. And the longer it takes to sort out all of these issues, the worse and worse it is for the general population. This creates more chaos in the already unstable situation of the creation of a new government. Within this chaos, the seeds of dictatorship are sown.
In one scenario, a moderate leader is the one who initially leads the charge for this new government, picked by both the more radical side and the less radical side, either as a compromise, or as a natural formulation of people falling in either on the more or less extreme of a set of views. This moderate leader is beset by the issues of the struggles of creating this new government, and people, both in and out of power, begin polarizing about these issues. And the side that wins out is the side that is willing to take instant action, to use the force and hard power at their disposal to force the new government institutions into existence, usually the more radical factions of the revolutionaries.
The radical leader takes power soon after these issues start to really hamper the new government, either through peaceful means, or through another revolution of the weak, infant government. This radical leader claims themselves to be the "true voice" of the revolution, the core of the passion, while painting the moderate leader as a lackey of the old government, holding back the "true" revolution. They are willing to use force to shape the new government into their own image, taking advantage of the passion of the revolution to obscure their dictatorial goals, whether intentionally or not.
This is only one scenario that could occur out of the situation created by a revolution. Regardless, the situation that creates the above outcome must be handled by the new government to prevent the rise of dictatorship and such. From the very moment a revolutionary force becomes a new government, it is directly at odds with revolution, yet the forces that allowed for a revolution to grow are still existing, and must be fixed to prevent further revolution. A revolution is directly causing instability, which the new government must then make stable once more. And given that the easiest way to make a situation stable is to force it to be stable, revolutions, especially wide sweeping revolutions that drastically change most aspects of government, tend to result in continuous turmoil and revolution eventually resulting in a violent dictator controlling the government by the end of it, as that dictator would be the one willing to use force and hard power to make the situation stable once more.
The best way to prevent such a scenario is to rely upon the already set traditions and institutions that grant stability to a government, but the communist revolutions of the 1900s generally involved overhauling most or all of a government's function, thus were unable to take advantage of the existing stable institutions.
Source: Revolutions: A Short Introduction by Jack Goldstone