Here's why:

* Oceanic Crust Formation: Basalt is the primary rock type that forms at mid-ocean ridges, where new oceanic crust is created through volcanic activity.

* Composition: Basalt is a dark-colored, fine-grained, mafic rock rich in magnesium and iron. This composition is typical of magma generated from melting of the Earth's mantle.

* Abundance: Basalt makes up the vast majority of the ocean floor.

While other igneous rocks like gabbro (a coarser-grained version of basalt) and peridotite (found in the upper mantle) are also present in ocean basins, basalt is by far the most dominant rock type.