Why do economies of Eastern European countries struggle?

The effect of communism.

Explanation:

Large portion of Eastern European countries used to be controlled by the Soviet Union, who imposed a strong government control over all resources through a Communists System.

Under their system, the government owned every factors of production that exist in the country. Making it impossible for the common citizens to own property. As a result, the incentives that the people have to work harder/contribute more to the economy basically non-existent. This eventually led to their downfall. After the Soviet Union was disbanded, the after-affect of their bad economic system still felt by their ex-members.

there is an explationation of this on a site called crash   course world history 1