Tricky question, best answered by a professor of the history of political philosophy but, alas, all you've got here today is me! (Mind you, I did pass 'O'-level history!) I am prompted to the question by the current state of China and also by my Russian e-pal, Alexey, who is facing the problem at first hand.
It seems to me that "that old common arbitrator, Time" is a huge factor. Consider English history from 1066 and the slow, halting, sometimes regressing, progress towards the sort of democracy we have now. Of course, the greatest advantage we enjoyed in this 'hit and miss' advance over the centuries was that we are an Island and thus avoided the constant interference of neighbours. That was not a condition enjoyed by most of our continental neighbours, although, given its huge size and inhospitable climate, Russia was almost an Island. Unfortunately, for them it worked the other way because their royal dictatorial leaders were free to make war on their neighbours and as we all know, there's nothing like a good war to help a regime increase its grip on the population. In fact, that is another feature of English 'isolation' in that for us, making war on countries across the sea was very difficult and very expensive but if it was undertaken it did not impinge directly on the English population and our political institutions.
Despite my negligible knowledge of European history, I would suggest that democracy only became a real possibility when the country concerned suffered a cataclysmic disaster. Germany springs to mind. Although it had a fledgling democracy after WWI it was easily swept away by Hitler and only in the ruins of their cities after 1945 did the Germans (irony alert!) under the diktat of their conquerors finally accept a full and functioning democracy. Similarly, the French went through the horrors of the revolution and Bonaparte's war, followed by yet another mini-revolution before they finally reached democracy.
So what of Russia and China today. Twice in the 20th century, Russia almost made the step-change into democracy. The first time it was slaughtered (literally at birth) by the Bolsheviks and the second time the very worst sort of mixture of naïve Americans and greedy bankers moved in and they were quickly out-foxed by the Russian 'kleptocracy'. Today, Russia is a dictatorship, not political but financial.
China, on the other hand has absolutely no experience of democracy. It is locked in the death grip of the Party, all of whose members only joined because, like racketeers everywhere, they wanted a share of the spoils. In a nation made up of a huge number of different ethnicities and traditions, to say nothing of languages, and therefor of a multiplicity of different needs, it is vital for the Communist Party of China (CPC) to grip every detailed facet of life in the entire country. However, to help sweeten their dictatorial pill they needs must grow the economy and share the wealth with the people. Thus, they have seized on western capitalism - without really understanding how it works, an ignorance shared by most westerners involved in capitalism! Like a Ponzi scheme, all will be well just so long as the wealth tap keeps flowing. However, the minute it slows to a trickle, or even worse, it stops, then that is the moment the chop suey hits the fan! I do not even wish to contemplate the catastrophe through which the Chinese people will have to suffer before they come out the other side and - perhaps! - realise that democracy, for all its inefficiencies and faults is infinitely better than dictatorship.
(Visitors expected any minute so apologies for any typos which I will correct later!)