Because nuclear power is still just thermal power. You need access to coolant. Ever wondered why a tsunami prone island like Japan decided to place a nuclear plant near the ocean? Doesn't that sound like a very stupid decision? Well, the reality is that this a necessary requirement for monolithic nuclear power plants. Rivers vary in their flow rates depending on the season. If the flow rate is too low there is less cooling capacity and subsequently you will have to turn the nuclear plant off, which will seriously hurt the economics of the plant. The only reliable cooling source is the ocean. Of course, if you have smaller plants you can live with less cooling.