The geographic requirements for pumped storage installations make it difficult. You need two large reservoirs with a good height between them. When you start thinking of places to put them, you start to see the problem. If there is a good place to build a dam at the top, then it's likely to be a flat plain at the bottom. Places with a nice valley at the bottom tend to have flat land at the top. Because it is hard to move large volumes of water long distances, the height needs to be good, to cut down on volume, and the distance between them has to be small, to keep piping to a minimum.
Add to this that it needs to be fairly close to both the demand and the supply, to keep transmission infrastructure costs and losses down, and you find that the opportunities to build pumped storage stations are, unfortunately, limited.
Then again, maybe I am looking at it with Australian eyes, living in what is a very flat country. And we have a very active green lobby, which is very good for some things, but makes it very difficult to, for instance, build a dam anywhere.