Here's why:

* Sedimentary rocks form from layers of sediment (like sand, mud, or shells) that accumulate over time. These layers compress and harden, trapping any organisms that were buried within them.

* Igneous and metamorphic rocks form under intense heat and pressure. This process would destroy any fossils that might be present.

So, while fossils can sometimes be found in metamorphic rocks (if they were originally in sedimentary rocks), they are most commonly found in sedimentary rocks.