Here's why:

* Formation: Basalt forms from the cooling and solidification of magma that erupts from mid-ocean ridges, where new oceanic crust is created.

* Composition: Basalt is a mafic igneous rock, meaning it's rich in magnesium and iron. This composition is typical of the magma produced in the upper mantle.

* Density: Basalt is denser than continental crust, which is why it forms the ocean floor.

So, while basalt can also be found in other locations like volcanic islands and continental flood basalts, its primary association is with the oceanic crust.