Here's why:

* Migmatites are metamorphic rocks: They form when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are subjected to high temperatures and pressures deep within the Earth's crust. This extreme heat causes partial melting.

* Foliation is a characteristic of metamorphic rocks: Foliation refers to the parallel alignment of mineral grains within a rock. This alignment is caused by the intense pressure during metamorphism.

* Migmatites have both igneous and metamorphic characteristics: The partial melting in migmatite formation creates light-colored veins (called leucosomes) of igneous material within the original, often darker, metamorphic rock (called melanosome). This creates a layered or banded appearance, which is a type of foliation.

So, while migmatites have aspects of igneous rocks (due to the partial melting), they are ultimately classified as metamorphic rocks because of their foliated structure.