Here's why:

* Sedimentary rocks form from the accumulation and cementation of sediments. These sediments can include fragments of pre-existing rocks, including metamorphic rocks.

* Metamorphic rocks form when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are subjected to heat and pressure.

* For a metamorphic rock to become an inclusion in a sedimentary rock, it must have existed before the sedimentary rock formed. The metamorphic rock was transformed, broken down, and then incorporated into the sediments that ultimately formed the sedimentary rock.

Think of it like this: You can't bake a cake with ingredients that haven't been made yet. The metamorphic rock is the "ingredient" that pre-existed and was incorporated into the "cake" (the sedimentary rock).