The most challenging part of any revolution is re-establishing order; just look at the French Revolution or the USA's original Articles of Confederation. Marx and Engels foresaw this in the Communist Manifesto:

If the proletariat during its contest with the bourgeoisie is compelled, by the force of circumstances, to organise itself as a class, if, by means of a revolution, it makes itself the ruling class, and, as such, sweeps away by force the old conditions of production, then it will, along with these conditions, have swept away the conditions for the existence of class antagonisms and of classes generally, and will thereby have abolished its own supremacy as a class.

Once the old class system is overthrown, communism won't magically happen. These two knew that it might very well require the proletariat to temporarily be a ruling class so that it might enact the final measures to wipe out the old state and establish the new. But People are people, so a dissolution of newly acquired power is tough, and the transition out of a ruling Communist party has never been the abolishing Marx and Engels hoped.

On a practical level, there's not one reason. The emergence of Communist states differs wildly over time and places: for example, the Russian Revolution of 1917 is very different from the imposition of Stalinism in post-war Europe. And of course each influences each other – the creation of the Eastern Bloc was essentially a product of events in Russia, and there is a vast amount of literature on the degeneration of the latter. (Was it inherent in Communism? Were the Bolsheviks naturally authoritarian? What of the changing class basis of the revolutionary regime?) That's a huge field that I'll not go into now, unless requested.

On a more theoretical level, viewing these as authoritarian regimes in general rather than specific historical events, I think Jacob Burckhardt nailed an important point about all revolutionary regimes:

The more recently power has originated, the less it can remain stationary. First because those who created it have become accustomed to rapid further movement and because they are and will remain innovators per se; secondly because the forces aroused or subdued by them can be employed only through further acts of violence.

Regimes that are born of violence therefore contain within an inherent propensity to continue in their use of violence, either because their usurpation of power has aroused powerful reactionary forces or because their support base (whether framed in class terms or as an elite 'selectorate') is not strong enough to reshape society without such coercive forces. Repression, and the construction of party dictatorships, therefore becomes a crutch that the new government falls back on.

This is particularly relevant in the case of Communism. Marxists would point out that (leaving aside arguments around 'stagism') the creation of a democratic and socialist society is theoretically impossible in a society that lacks a large working class. Lacking this support, the implementation of a communist programme, and the survival of the regime, therefore requires the use of repression in predominately peasant agrarian countries. But this is where historical reality keeps butting in – can the emergence of worker soviets in Russia really be compared to the Derg's military coup in Ethiopia?

This is a tricky question and you will get diffrent responses. There ar ea number of thinkers who have thought about this.

First I would want to point out that some communist WANT dictatorships. Lenins innovation was the establishment of a core party that would take leadership of the country. Now as one might expect, a small core of people getting all the power are unlickly to voluntarly get ride of there power, and those in this groupe that want to move towards democracy might find themselfs dead. So in these cases I think there is absolutly no suprise.

There is another theory that you simply can not have socialism without dictatorship. F.A. Hayek layed this out in his book "Road to Serfdom". This book is often misunderstood and hard to sum up. His core argument is that, if you really on state policy to fix every problem, you will head into dictatorship. This is because the socialist idea of "consensus" just does not work over a large amount of diverse people. You will have to go with the majority and not the true "consensus" socialist hope for. Now that in itself is not enougth. Because the democracy is expected to solve ever more complicated processes more and more power is given ower into burocracys and the democraticlly elected leader only say yes/no to some plan designed by a planning burocracy. If you in a socialistic state to expect everything to be handle this way, the democracitc porssess will lose its power. (Read Chapter 5 "Planning and Democracy"). The books explores every aspect in quite some detail.

I dont here want to make the argument that Hayek was right, its one explaition, why we dont have traditionally socialist (as imagened by early socialist) societys today. I think Hayek underestimated how far you can go democraticlly if you do it right, we have states today that have around 60% of GDP go threw government. However we also see that if it goes over that it tends to bounce back. The "if done right part" seams to be importent. For example redistribution of income does not really controll in the same way that nationalising the production of industry would.

I would recommend the book, it really does contain a lot of clear thinking about these issues, even if you dont agree with hayek overall. Also, read the book not as a prediction but more like 1984, a warning to the society the auther lived in.

Another book worth mentioning is the "Pictures of a Socialistic Future". It is much older (1891) and thus predates many of the revolutions and real socialism. However when you read it, you will be shocked in how accurate to later development it is. So accurate that it must be more then a coincident, the auther did allready understand some of the issues that real socialist experiances has had.

I think the simplest and most easy explaition is that, if a revolutionary elite takes absolute power they want give it back. Ideals dont matter because:

Power corrupts; absolute power corrupts absolutely Lord Acton