Mostly it's that solar doesn't work at night. That means you have to use batteries, which are impractical to store more than a certain amount of energy, after w

Mostly it's that solar doesn't work at night. That means you have to use batteries, which are impractical to store more than a certain amount of energy, after which you need another very large and stable energy source. So a nation-state that can't go dark must have a constant load source, such as nuclear, hydro, or coal. There's also limitations of geography, industry, production capacity, and other issues.