Here's why:

* Sedimentary rocks form from layers of sediment: These layers can be sand, mud, shells, or other materials that settle at the bottom of oceans, lakes, or rivers.

* Fossils form when organisms are buried in sediment: Over time, the sediment compacts and hardens into rock, preserving the remains of the organism.

* Igneous and metamorphic rocks are unsuitable for fossilization: Igneous rocks form from molten material, and metamorphic rocks are changed by heat and pressure. These processes would destroy any fossils.

While most fossils are found in sedimentary rocks, there are rare instances where fossils can be found in volcanic ash or other types of deposits.