And local factors--people just assume that if necessary they can drain the reservoir (or greatly lower the level) to run the turbines to cover for a lack in solar/other output, but in the US as soon as you put in a dam you'll get home owners and property owners around the newly created reservoir fighting tooth and nail to maintain high water year round for aesthetics and recreation.

There are ways to minimize this of course, like building giant storage tanks, but dammed rivers are going to (usually) dwarf man mad retention pools.