Before oil was even a thing that region was incredibly volatile. It is the cradle of civilization after all, so I guess it was always bound to be a powder keg.
It has only become volatile after the Ottoman Empire collapsed. Was quite stable and safe before. It's just they didn't have nations before, were colonised and controlled by Ottomans while in Europe we had nation-states and nationalism since ~17 century and had enough time for cleansing to organise mono-ethnic, cohesive states. While even that, was only finished decisively by post-WWII "population transfers" (which was nothing but ethnic cleansing sold as a good thing).
Ethnic states are a things, albeit quite debatable since ethnicity is an entirely contingent and subjective construct. Monoethnic states are the dream of genocides, not ever a reality on the ground (doesn't matter how you define ethnicity).
Well where in a state there's no word in a language to differentiate between a citizen and someone of a title ethnicity, i can call it an ethnostate. Say Armenian or Polish: there's no 2nd word in their languages for that. Because there are so few citizens who aren't of title ethnicity, and they are so marginalised and subject to such a quick assimilation, there isn't even a need to bother inventing a word.
If there is such a word say, "Kazakh" vs "Kazakhstani" or "Russkiye" vs "Rossiyane", existence of such a word shows that this state is an empire.
Sometimes the "ethnic" word simply doesn't exist, say in "Canadian" or "American" for U.S.: it's only citizenship, there's no ethnicity at all - neither the word nor substance - showing a "melting pot" country.
That's as simple as having vs not having a title ethnicity in the country. If there's no title ethnicity it means there's just citizenship, no one subjugates another: it's a melting pot. Kazakhstan in particular, in any case, is moving towards an ethnic state, as every other ethnicity except Kazakh has very low birth rate there so in a few generations it will become homogenous.
They are still just starting on it.