Historically? Raped by Hun, Magyar, Mongol, Ottoman invasions, and by the Austrian and Russian empires. Take a comparison that is rather indicative of the developmental differences: France's peasant population is 17% by the end of WW2, Hungary's is 60% at that time, Romania's is around 70%.
Then comes the Marshall plan for the West while Hungary, Romania, and GDR are paying war reparations in the East... Around 60% of industry and vital infrastructure was destroyed in Poland and Hungary in WW2.
This is where socialism starts, bringing with it massive urbanization, production of agricultural technology, industrialization, a standard of living that was closest to the West's in this region's history. Then comes the regime change, reversion to capitalism, selling off the region's MoP, land, infrastructure, and resources to the West for literal pennies - a lot of which they just blatantly closed after buying, dominating these markets with their own products, hence wast unemployment, social ills, etc. in the nineties.
After joining the EU these markets get further dominated by Western finance capital. Brain, workforce, and youth drain to the developed West is taxing enormously these economies, contributing further to population aging, destruction of the social safety net and so on.