The root issue is that a lot of "green" generation systems have functional limitations.
Solar is consistent, but only generates when the Sun is up (obviously). The advantage there is that most of our energy needs are during the daytime, so as a supplement to a power grid, it's actually really effective, to offset that bump of daytime usage, even without worrying about battery storage.
Wind is, of course, dependent on the wind speeds. It's a great benefit to diversify the grid and provide local generation where possible, but it can't be a primary/emergency source.
Hydro is much more reliable, but you still run the risk of long-term droughts forcing you to shut the plant down and hoard the water supply, particularly if the same reservoir is used for drinking waters.
All of those play against each other, of course, but there's still a need for SOME clean, fuel-based generation system, even if it's only used as a stopgap for when green sources aren't providing enough, to prevent brownouts. Nuclear fills that gap pretty well, and if nuclear's out of the picture, natural gas can be an
okay fit. Not ideal, but better than alternatives.
I know I didn't mention geothermal or tidal, but those are more consistent but also regionally focused; they're great if they're an option, but a geological cold spot in a landlocked area can't use either.