Here's why:

* Solubility: Limestone is primarily composed of calcium carbonate, which is soluble in acidic water.

* Formation: Rainwater picks up carbon dioxide from the atmosphere, becoming slightly acidic. This acidic water seeps through the ground and dissolves the limestone, creating cavities and eventually large caverns.

* Other Rock Types: While caverns can sometimes form in other rock types like gypsum or marble, limestone is the most common and readily dissolved rock.