One explanation would be that after the Black Death, which made agricultural labor a much scarcer factor of production, landowners and peasants engaged in a set of class conflicts over who would control labor. In Western Europe, the peasants won (this round) and tenures lost most of their labor obligations and mobility restrictions; in Eastern Europe, the landowners won and reinstituted and intensified their control over the peasantry. If you want to know more about why each side won where it did, check out Robert Brenner's essay "Agrarian Class Structure and Economic Development in Pre-Industrial Europe," (Past and Present 70: 1976)

The modern Russian state is the descendent of the Duchy of Muscovy, a cute little entity (initially comprising only Moscow and its environs) surrounded on all sides by rival powers. The peasants worked the land and gave their grain to Moscow, which funded the military that kept them safe from neighboring powers.

Not content with being surrounded by enemies, Muscovy started expanding. A lot. By the 18th century, Russia (now ruled by a Tsar) extended from Karelia on the Baltic Sea to the Kamchatka Peninsula on the other side of the continent.

The thing is, in order to supply the enormous military machine required to conquer all this territory, Russia needed to dedicate all the economic surplus of its peasantry to funding it. As a result, you have massive amounts of wealth and resources and power all being sent to Moscow (and later St. Petersburg). These resources were not put towards the purpose of developing Russia's provinces; instead, local landlords and barons supplied Moscow with an annual tax (usually in the form of grain) and conscripted peasants for Moscow's armies.

As a result, there was never much in the way of a civic culture in provincial Russia. In contrast to places like Europe and the Levant and China, where nascent market economies distributed societal development over a more diffuse area, Russia never really moved on from the feudal system that supported its all-important military. Local administration was devolved to Councils of Nobles. Aristocratic landlords would get a world-class education in Moscow or St. Petersburg, then go back and manage their estate for 50 years in the hinterlands until they died. There was never any incentive or reason to lift the peasants from their illiteracy and poverty, since there was no society for them to become a part of - just the meeting hall of their village and the landlord who demanded some bushels of wheat at harvest time.

So the peasantry never really escaped this SUPPLY THE CENTER strategy that characterized the Russian state for hundreds of years (see how this evolves into a centrally-planned Soviet economy?). As a result, they socioeconomically stagnated until serfdom was abolished by Tsar Alexander II in 1861.

tl;dr: Russia spent all its resources funding the military that allowed it to conquer so much territory. As a result, it never developed its economic hinterland.

Source: I'm writing my final paper on this

Serfdom lasted much longer in Imperial Russia for a myriad of factors. For one, serfdom was almost abolished by Nicholas I. Correct me if I am making a mistake, but Nicholas I had abolished serfdom in "test nations" like Latvia. What he saw was that with the abolishment of serfdom came the probelm of what would they do with all the people that are now free and out of "work". He was against serfdom, but the higher ups saw it as almost a burden to try and abolish serfdom. It also has to be said that serfdom lasted longer because Russia as a society had kept the feudal type system for much longer than Europe had. When the Romanovs came to power after the Time of Troubles Russia was still more or less a nation with very closely related city states in terms of the boyars controlling the major city centers. It wasn't until Peter I (the Great) did Russia start to modernize itself in the 1700s. It was one of the pioneers of the first major conscripted Army in Europe afterall.

I think something also has to be said about the Enlightenment period itself. Russia was hesistant to let go of the serfdom system because it would allow the serfs to have much more freedom and possibly revolt. Not that revolts didn't happen because they did during this time like Pugachev's Rebellion.

Overall, Russia had always been a nation that was late to the game in comparison to Western Europe. Sorry if what I am saying is simplistic at best; hopefully someone with a much more intimate understanding of serfdom will reply :) Sources: A semester of Russian History at West Point.