The first thing to observe, is that, under Communism, Bulgaria was a favoured son of the Soviets.The Romanians were more a prodigal son, as their foreign policy was rather eccentric (leading to Ceaucescu being knighted by the Queen).
Bulgaria's tourism industry was built on Eastern Bloc countries, particularly Russia and the GDR. After the fall of Communism that expanded to include the whole of Germany, and other countries like the UK
Bulgaria is also much better at marketing its ski resorts. Romanian ones are primitive by comparison.
Recent figures also make interesting reading; 70% of visitors to Romania head to Bucharest, mainly as a business destination.In Bulgaria, Varna, Burgas and Plovdiv pull in a much larger share.
The Romanian economy is a much bigger success story that the Bulgarian one.
However, tourism seems to be an exception to that rule.
Romania has the historic, and beautiful, Transylvania region.But the 'Dracula Tourism' trails have only just been opened up. There are also dozens of beautiful castles such as Bran and Peles, historic towns such as Sinaia, Sibiu and Brasov, with rich Saxon heritage. Bulgaria does not have this (though one has to mention Veliko Turnovo and Plovdiv).Both countries have a huge amount of Monasteries.
It's not even that it is difficult to get flights, Ryan Air and Easyjet fly to all major cities in Romania; such as Cluj and Timisoara.Romania has the Danube Delta and skiing too.
There is still under-developed potential in both countries too.Spa and Wellness Tourism is in its infancy and yet Bulgaria has hundreds of Mineral Springs.
I think the casual observer, has to put the differences down to very poor marketing. Romania needs to sell itself far better.Why is it that there is not one golf course in Romania and 7 in Bulgaria ?, but more Romanian golfers than any other single group playing in Bulgaria ?.No really convincing answer.