Here's why:

* Composition: Oceanic crust is primarily composed of basalt and gabbro, which are mafic rocks. These rocks are rich in dense minerals like iron and magnesium.

* Thickness: Oceanic crust is much thinner than continental crust, averaging about 7 kilometers thick. This thinner layer contributes to its higher density.

In contrast, continental crust is primarily composed of granitic rocks, which are less dense than mafic rocks.

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