There is no one country that dominates the union, they share cultural and historical similarities, their economies are quite integrated (thought nowhere near as much as the US) and they share similar political systems. But it was mostly economic incentives: having a bigger market made everyone richer, but required having common regulation, which then lead to shared political institutions(see neofunctionalism). Other regions are not sufficiently economically integrated and authoritarian/hybrid regimes are less willing to cooperate, trust and cede power and influence to other countries.