They have infinite loading cycles, they are rather bulky, basically a fluid medium is charged and discharged. This allows for the storage capacity to easily scale (the generator remains, the tanks increase).

In addition the storage fluid can be rather friendly substance from a environment perspective (some use drinkable salty water). And cheap. And if you have a old pool or a old oil tank in your house you are basically halfway there regarding installation. Its basically chemical storage of energy, allowing for the (re)use of preexisting installations.