The Bulgarian use of sunflower oil for cooking is a new thing, it arose when the Soviets closed the border with Greece, and also destryed the small businesses that produced peanut oil domestically. I think thecause peanut oil roduction is less amenable to the Soviet fetish of mass production and they saw it as more bourgeouis.

Historically of course Bulgarians lived in palces where olive oil was produced but after the Greek conquered them they were driven out in the early 20th century, although in the very southern parts of what is now Bulgaria it's possible to grow olives, but it used to be economically pointless because trade was free and you could grow them more easily even more to the south before that, and commercially growing olives takes a lot of years to set up.

But sunflower oil is kind of an odd thing to use in Bulgarian cooking, if you want to be traditional about it -- although it's very healthy.