There are a lot of things that must be considered before classifying a country to be a developed country or developing country. Factors like poverty rate, the revenue or income of the country, the unemployment rate, the crime rate, the GDP per capita. All these, when used to rate Bulgaria, it simply means Bulgaria is a developing country.
However, according to the report published by the human development index, it indicated that Bulgaria falls into the 51st position out of 195 countries. This shows and explains more about why Bulgaria can be regarded as a developed country. Looking at the GDP per capita of Bulgaria, you would see that in no time, Bulgaria would join the league of best performing economies in the world.
Because Bulgaria despite its presence in the EU, it has not made a proper use of that opportunity, many countries who joined the EU after Bulgaria have been doing well economically.