Marx was an advocate of the revolution of the proletariat in taking over the bourgeoisie power. To achieve this, Marx would advocate that the working class of each country should stage a revolution where the proletariat would achieve power. To answer why Marx thought that Russia was “Premature” to engage in a revolution, it is important to combine the social structure of Russia at the time, as well as Marx’s idea of what constitutes and proletariat from a serf.
Russia changed in the late 19th and early 20th century to adopt more progressive social policies, like the creation of the 1906 constitution, the State Duma, and most importantly the emancipation of serfs in 1861. With the liberation of serfs to the workforce as free agents, most were stuck with their life of farming. With little knowledge of the outside world and no formal education for themselves, they were thought to be useless by Marx. Eventually these serfs traveled to the city centers where they were employed as factory workers, but even here Marx makes a point that no matter what service a peasant takes, whether he be a craftsman, buy out his own land or takes his owners land by force, and no matter which course he takes, he will still be part of the bourgeois movement.
With this mindset, Marx differentiates serfs from proletariat due to the fact that the former is a player in the competition, not part of the masses that rule the system of labor. With this in mind, he argues that there is not a sizeable force, considering that over two-thirds of the Russian population were peasants. This compared to England, and a lesser extent to Germany, whose economy had passed the era of industrialization that created the army of workers that can engage in a revolution and take over the means of production.
One of the important factors that Marx did not take into account was the unequal distribution and density of workers that crowded the main cities of Russia. While the proletariat class was considered small, it was concentrated in strategic social, and political centers throughout the country. With the sudden increase of peasants entering the workplace, they moved from a setting where they lived in small villages, unknowing to the world around them, to sprawling cities where the ideas of revolution were taught in evening classes, library, and spread throughout the workplace. The new class spawned by the increasing immigration of peasants into cities grew a social and political awareness. They organized themselves in worker councils throughout Russia, also called Soviets.
Maybe Marx was not wrong about Russia being least likely to enact a revolution in the time he wrote The Communist Manifesto, because at that time Russia was less industrialized than in 1917, and it required two revolutions, the one in 1905, and in February of 1917 to change the country into a bourgeois government, and from there to start with the proletarian revolution. The Provisional government's decision to stay in the war, and simply appease the Soviets did not help them move the minds of the Russian people, and with a slogan like “Peace, Bread, and Land” it was not a difficult decision for the proletariats to side with the Bolsheviks