Why hasn't there been/can't be a democratic communist state?

To actually pull this into the realm of history, a vast amount of ink has been split over the past century as to the reasons of the failure of the Russian Revolution to create a democratic and socialist* society. Yet there's not much agreement on the cause or even course or this 'degeneration'.

Ignoring outright political tracts, this literature tends to break down along three lines. (Simplifying hugely, naturally.) These are largely based on the weight one assigns to either ideology or circumstance:

The 'totalitarian' school focuses on ideology and political institutions to argue that Bolshevism/Communism is an inherently authoritarian ideology. It either stole into power on the back of terror or is the result of some innate Russian conservatism. There is no real dilemma in this scenario: the Communists succeeded in building the totalitarian state they wanted all along. Richard Pipes is probably both the most extreme yet prominent example of this.
Fatalist or 'Menshevik' histories argue that while there was a genuine population and push for democracy in Russia, this was thwarted by the conditions in Russia at the time. That is, hopes of a democratic and modern society were thwarted by its economic and cultural backwardness, of which the Bolsheviks' popularity was a manifestation. Think of the post-October memoirs by Mensheviks like Sukhanov or modern historians like Orlando Figes (the clue is very much in the name of his A People's Tragedy).
The 'Revisionists'. It's a very broad term to describe the shift towards 'from below' studies of the revolution from the 1970s onwards. Broadly these historians reject the totalitarian thesis, emphasising the popularity and mass character of the Bolsheviks in 1917 and locating the failure of the revolution in a combination of circumstances (eg deurbanisation, economic crisis, Civil War, etc) and the Bolshevik response to these.
(As an aside, Trotskyist histories tend to place blame on both the failure of the revolution to spread to more advanced European nations, which is a legitimate placing of the revolution in an international context, and the rise of Stalin, which is scapegoating.)

I've got a lot of time for the revisionist bracket, if only because it's a lot more nuanced than earlier efforts by émigrés or Cold Warriors. It shows, for example, that we can certainly talk of 'failure': there were genuine, if deeply complex, democratic aspirations in 1917 and the Bolsheviks themselves were highly conflicted as to how to salvage 'their' revolution. (Party, State, and Society in the Russian Civil War summarises many of these themes, looking at the social impact of the repeated crises of 1917-1922 and the Bolshevik responses to these.)

And, to return to the original question, this is just one example Communist nation. Yet even here, one of the most studied events in history, there is no one answer. Trying to extrapolate these arguments across a host of other countries (from Cuba to China) is, IMO, the realm of politics rather than history.

*Socialist in the Marxist sense. To be clear on the terminology:

The Bolsheviks were socialist in that they were a Marxist party that belonged to the international workers' movement.
They were communist because they were on the radical left of this socialist movement. (And were Communist because, from 1918, they called themselves the Communist Party).
Their view of the future had two stages: a socialist society in which the proletariat was in power (ie the 'lower phase of communism'); this would lead to communism, a classless society of abundance with no state or money (ie the 'higher phase of communism').

[Note to self: Type faster.]