Two issues. First is that pumped storage requires a significant change in elevation. In other words, you need a good-sized kill or mountain nearby. So maybe good for Colorado, not so good for Kansas.

Second, reservoirs require water. And you may have noticed that in many places water is in short supply, and that many existing reservoirs are already dangerously low.

IOW, it's a very localized solution to the problem and one not suitable for may areas and environments.