So much this. Among other causes: Poland has a population of more than 35million people, so they were able to export a lot of workforce - which then generated significant remittance income, a fundamental driver of capital creation in poor economies. Bulgaria is less than 7 million, they obviously could not do it on the same scale; they also suffered from blocks on free circulation of workers, imposed on Romania and Bulgaria on accession precisely on the back of the experience with Poland. These blocks were only lifted about 10 years ago - and significantly increased the lag they already had versus the Baltic states (which were allowed to join much earlier, largely because of strategic German interests that did not apply to Romania and Bulgaria).
