I thought that yogurt is widely used in all formal Ottoman lands. And it is called yoğurt because milk becomes sticky (yoğun), thus yoğurt in Turkish.
"1625 S. Purchas Pilgrimes II. ix. xv. §9. 1601 Neither doe they [sc. the Turks] eate much Milke, except it bee made sower, which they call Yoghurd."
(See also[1]) I suppose it's possible that Bulgaria got yogurt during Ottoman rule in the 14th century, and that the research in the early part of the 20th just happened to be there. But it seems more likely to me that any group that kept animals for milk would have fermented the milk to make it last longer, and that knowledge would have traveled all over Eurasia.
1: https://www.bonappetit.com/test-kitchen/ingredients/article/...
Indeed it was used in all formal Byzantine empire -- later "Ottoman".
But it was known far beyond that: "The cuisine of ancient Greece included a dairy product known as oxygala (οξύγαλα) which is believed to have been a form of yogurt" (Wikipedia), and even further to pre-history (5000 BC)
Modern Greek has seen a great deal of lexical replacement. It uses new words for things like "bread" and "water" that are different from what were used in the Classical era, even though bread and water have never changed. There are even cases where Ottoman vocabulary has replaced native Greek words. It is not inconceivable that the word for oxygala changed but the dairy product didn’t.
Citation needed.
>If yoğurt was oxygala Greeks would continue to call it oxygala.
That's not how language always works. Greeks have adopted foreign works for things they themselves had in the past (with their own words), and also use ancient Greek words for things that were imported much more recently. Even something as basic as water has changed name from ancient Greek times.
Words have complex itineraries and adoption cases. Sometimes a newer foreign word is adopted because that's when a product gained more popularity (and the older one has been forgotten), or merely because it's thought as "more modern" to refer to it by that name.
There are is even "reborrowing", when an ancient words is re-imported, centuries or millennia later, after it has been adopted and slightly altered abroad.