Here's why:

* Calcium carbonate (CaCO3) is the primary mineral component of limestone.

* Limestone forms through the accumulation of skeletal fragments from marine organisms like corals, mollusks, and foraminifera. These organisms use calcium carbonate to build their shells and skeletons.

* Over time, these fragments are cemented together, creating a solid rock mass.

There are other types of rocks that contain calcium carbonate, but limestone is the most common and specifically defined by its high calcium carbonate content.