The basic idea is that you are storing energy by lifting something heavy. The more mass you can lift, the more energy it can release as it falls back down. Rock density varies, based on the type of rock, but it is generally 2-3 times heavier than water for a given volume. The limiting factor for water energy storage systems is how much space it takes to store all the water. So the big chunk of rock on top of the water column means you can store more energy in the same space.