Geography olays a big role there. There are many places where the capacity of hydro needed requires more than is environmentally safe to dam. We would have to have gigantic mega projects to build new lakes and resivoirs. (Much slower and more labor intensive than building nuclear plants)

If nuclear doesn't fill the gaps it will be fossil fuels.In the current moment, an 'always no matter what' anti nuclear stance is just a pro oil and coal stance.