Here's why:

* Formation: Limestone is formed primarily from the accumulation of the shells and skeletons of marine organisms, which are composed of calcium carbonate.

* Abundance: Limestone is found all over the world, and it makes up a significant portion of the Earth's sedimentary rocks.

* Variety: Limestone comes in many forms, including chalk, travertine, and dolostone.

While other chemical sedimentary rocks like rock salt and gypsum are important, limestone's widespread occurrence and significant volume make it the most abundant.