I am very surprised that no ine has pointed the another major reason for Bulgaria yet - the Balkan Wars, specifically, the Second (Inter-Allied) Balkan War. 

After 1913, the imports of olive oil from Greece and the Ottoman Empire ceased, leaving Bulgaria without consumer cooking oil - imported olive oil being the main one prior.

That, coupled with the trauma of the war, the whole "cheater allies" mentality, and World War I that soon followed, lead to a massive shift to growth of sunflower (around the entire country and especially in the Dobruja region) and production of sunflower oil. Following the Soviet occupation and  Communist takeover of the country, sunflower oil was additionally promoted as the "proletariat" substitute. 

Gradually, it became a staple of Bulgarian cuisine over the past century.

It's hard to overstate today just how many aspects of modern life in Europe (not just Bulgaria) are consequences of the major industrial wars of the early XXth century.

On sunflower oil itself, it's not bad at all. It's not as healthy as olive oil but it's far, far better than rapeseed oil. Go unrefined for salads and seasoning of raw foods, refined (higher smoke point and longer expiration period) for cooking. It actually tasted quite good, especially high quality unrefined ones.