Here's why:

* Igneous rocks are formed from the cooling and solidification of molten rock (magma or lava). Basalt is formed from the rapid cooling of lava flows.

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

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

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