Good question! This is very much a theory-based question, and you'll get different answers based on who you ask, but I think the most compelling answer is rooted in the differences between institutions in Western and Eastern Europe.

According to economists Daron Acemoglu and James Robinson, in the 14th century, landlords in Eastern Europe were marginally more powerful than their counterparts in Western Europe. They had more rights and more consolidated holdings; also, potential rivals to their power, like towns and peasants, were weaker and less organized. But, in general, the economic and political situations in Eastern and Western Europe were similar.

When the Black Death hit in the middle of the 14th century, both Eastern and Western Europe were devastated. In England (birthplace of the Agricultural and later the Industrial Revolutions) and much of Western Europe, the surviving peasants were able to take advantage of the drop in supply in the labor market to demand higher wages and more rights. However, in Eastern Europe, where landholders were more powerful, the landlords merely took over more land and increased taxes on the peasantry.

This began the Second Serfdom, a period when the rights of Eastern European serfs actually decreased. According to Acemoglu and Robinson, this was a critical juncture in history. While the Black Death allowed Western Europe to take the first steps towards being a market economy, with rewards for innovation and labor, Eastern Europe actually took several steps back.

Here's a quote from their recent book, Why Nations Fail:

Though in 1346, there were few differences between Western and Eastern Europe in terms of political and economic institutions, by 1600 they were worlds apart. In the West, workers were free of feudal dues, fines, and regulations and were becoming a key part of a booming market economy. In the East, they were also involved in such an economy, but as coerced serfs growing the food and agricultural goods demanded in the West.

Wow, no offence but these answers are missing a very important aspect.

The richest countries pre-Columbus were Venice and the Ottoman Empire which controlled the spice and silk trade from the East. Once Western European counties established trade and colonies in Asia and the Americas, they generated tremendous wealth. Eastern Europe didn't such an easy access to wealth generating ventures.

The Ottoman Empire is a mainstream justification as to why Romania wasn't as well developed as Western Europe, according to my professor. The country did see a short term period of prosperity after the fall of the Ottoman Empire, and even gained a large amount of territory from the dissolution of Austria-Hungary at the end of WWI (even if Romania got its butt kicked during the war). Romania even had its own monarchy for a short period of time.

This of course ended after World War II when the country became a Socialist Republic installed by the USSR. Romania was more autonomous than other satellite states but a wealth of its natural resources and the bulk of important decision making was dependent on Moscow. By the time Ceausescu became president, the country gained even more autonomy but Ceausescu's social and economical policies in the late 70s and early 80s were devastating to its economy.