Bulgarian here, studied and lived abroad, now back to Sofia.
The answer to the question "why this is the case" is...it makes sense a lot of sense. Bulgaria joined EU in 2007 and right after that the crisis hit Europe and the world. Wages at that time were probably 25% of the average in EU, suddenly no restrictions to move (that's the point of Europe right?), people wanted to try how it is because they've only heard and read how much better it is, some people took loans before the crisis, etc. But it's the same reason why people from East Germany move to West Germany, or from poorer states in US to California..
That said, I believe it's a good place to work and live and I claim it's reversing the trend slowly. I can bet there are more people coming for the IT/engineering jobs than leaving nowadays, many foreigners living in the big cities, you can afford to buy an appartment in 5 years, go to ski in Bansko and to Greece/Croatia/Black Sea for the summer. Income tax is at 10%, food is good, there is interesting companies, VC and acquisitions. Unemployment rate is 4-5% and if you're halfway capable you can get a job in no time.