It takes a lot of money, planning, and technical know-how to build a nuclear power plant, especially a safe one. It isn't like a new nuclear company can just pop into existence, and start offering reactors for sale.

Traditional nuclear reactors are, therefore, a technology that requires a lot of centralization to implement. Only nation-states and huge corporations can assemble the resources to construct them.

Compare that to wind or hydro-electric power. You can build a generator with some wire and magnets yourself, so you could call them more decentralized.

This might be changing with modular reactors, I don't know.