it's the region of confluence of all major trade on the continent. it's like water in a river bed, it settles in a middle and then as it keeps running through that area, it keeps dragging sediment, making the river bed even more firmly settled. that's exactly the center of western europe, the core of it and outside that area are the peripheral areas that all tended to ship their goods towards that concentrated marketplace in the core.  and that area inherited a lot of culture and treasure from rome and all its avatars (frankish empire, hre, byzantine empire) and it was a sort of incipient form of regional economic integration. like eu before it became a thing. no it wasn't about ethnicity.
Romania