Here's why:

* Most caverns form in soluble bedrock: The most common way caverns form is through the dissolving action of acidic groundwater on soluble rock types like limestone, dolomite, and gypsum. This process is called karst topography.

* Other bedrock types can also form caves: However, caves can also form in other types of bedrock through different processes:

* Lava caves: These form when lava flows leave behind empty tunnels.

* Sea caves: These are formed by wave erosion along coastlines.

* Glacial caves: These form when glaciers carve out underground channels.

Therefore, while caverns are most commonly associated with soluble bedrock, they can form in other rock types as well.