Here's why:

* Igneous rocks form from the cooling and solidification of molten rock (magma or lava). Feldspar is a very common mineral found in igneous rocks like granite, basalt, and gabbro.

* Metamorphic rocks form when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are transformed by heat, pressure, and chemical reactions. While feldspar can be present in metamorphic rocks, it's not formed during the metamorphic process itself.

* Sedimentary rocks are formed from the accumulation and cementation of sediments. Feldspar can be a component of sedimentary rocks, but it's usually in the form of weathered grains from pre-existing igneous or metamorphic rocks.

So, while feldspar can be found in sedimentary and metamorphic rocks, its origin lies in the cooling and crystallization of magma or lava, making it an igneous mineral.