Here's why:

* Composition: Oceanic crust is primarily composed of basalt and gabbro, which are denser than the granite and other rocks that make up the continental crust.

* Thickness: Oceanic crust is thinner than continental crust, which also contributes to its higher density.

This density difference is a key factor in plate tectonics, as denser oceanic crust is subducted beneath the less dense continental crust at convergent plate boundaries.