Communism. It's pretty much same reason in whole Eastern Europe. Some countries managed to recover faster for various reasons whether it was more useful industry which was pretty much centrally planned in whole Soviet block from Moscow, so some countries where more industrially orientated and other remained pretty much agriculture only.

Second important reason is geographical location, especially presence of wealthy neighbors - compare Czechia with Germany and Austria as neighbors to Romania whose economically best performing neighbor is Hungary (which is economically worse off than Czechia itself).