Here's why:

* Igneous rocks form from the cooling and solidification of molten rock (magma or lava). The intense heat involved in their formation would destroy any existing fossils.

* Metamorphic rocks form when existing rocks are transformed by heat, pressure, or chemical reactions. While some fossils might survive the transformation process, they often become distorted or unrecognizable.

* Sedimentary rocks form from the accumulation and cementation of sediments, which can include the remains of organisms (fossils). The relatively low temperatures and pressures involved in their formation allow fossils to be preserved.

Therefore, sedimentary rocks are the primary source of fossils.