Here's why:

* Igneous rocks form from the cooling and solidification of molten rock (magma or lava). The intense heat involved in this process destroys any existing organic material.

* Sedimentary rocks are formed from the accumulation and cementation of sediments, which can include the remains of organisms. This makes them the most common place to find fossils.

* Metamorphic rocks are formed when existing rocks (igneous, sedimentary, or other metamorphic rocks) are transformed by heat and pressure. While some fossils can survive the metamorphism process, they are often distorted or destroyed.

So, while fossils can sometimes be found in metamorphic rocks, they are much more likely to occur in sedimentary rocks.