Here's why:

* Igneous rocks form from the cooling and solidification of molten rock (magma or lava). Diorite is formed from the slow cooling of magma deep within the Earth's crust.

* Metamorphic rocks are formed when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are transformed by heat, pressure, or chemical reactions.

* Sedimentary rocks are formed from the accumulation and cementation of sediments, like sand, mud, or organic matter.

Diorite doesn't fit the definitions of metamorphic or sedimentary rocks. It's formed directly from cooling magma, making it an igneous rock.