Russia was weakened by war. Its political immune system was suppressed and disease was free to enter the body politic.The Russian revolution was the first success of several revolutionary movements of the late 19th and 20th centuries seeking to bring about communism. The institution of the Tsar was oppressive.
The table of ranks of elites rendered ordinary people nobodies beyond the first 50 ranks. An international movement sought any place available to revolt. The revolutionaries moved about. Marx, Lenin, Trotsky and others journeyed from nation to nation as the royal establishment kicked them out or hunted them. Russian royalty made a fatal mistake in the social contest between revolutionaries and aristocrats; it got involved in a war with Germany and the Tsar was not a good military leader. His pre-emptive attack on Germany led to great German advances against Russia. Germans sent Lenin in a sealed train to lead a revolution and sign a treaty with Germany that gave away most of the Ukraine and other land.Russia had anti-tsarist revolutionary movements for a very long time before the October Revolution.
There had been experiments with democracy a little, briefly. It was possible that a constitutional monarchy might have evolved something like that of Britain given enough time, yet the pressure of the first World War that reorganized much of Europe was sufficient to bring more force to the fault lines in Russian leadership.The Spanish philosopher Ortega y Gasset wrote a book named The Revolt of the Masses-published in 1929–30 that anticipated the restiveness of European society because in part, of technological changes and mass communication, transport etc. The work anticipated the 2nd World War, though not exactly concise in anticipating the rise of Hitler and the 3rd Reich.I believe it worthwhile to mention that several nations did join the communist camp through force after the Second World War.
War is a traditional means of bringing communist-socialist systems into being. Usually the desire is to remove an oppressive ruling elite. Then at the conclusion of the war the choice of the leadership is for communist dictatorship rather than democracy not only for ideological reasons, but for practical reasons too; suppressing counter-revolutionary werewolves is easier with dictatorship than ballot boxes.In places like Vietnam and China communism prevailed yet later moderated itself with some evolution toward market political economy. The perennial war of ordinary people to rid itself of oppressive ruling elites never ends. Democracy and free markets seem the best remedy.