Here's why:

* Sedimentary rocks form from the accumulation and cementation of sediments, which are particles of other rocks, minerals, or organic matter.

* Bituminous coal forms from the accumulation of plant matter (mostly trees) in swampy environments. Over millions of years, the plant material gets buried under layers of sediment, subjected to heat and pressure, and transforms into coal. This process is called coalification.

While some metamorphic processes can occur during the formation of coal, the primary process is the accumulation and transformation of organic matter, which is a characteristic of sedimentary rock formation.