Here's why:

* Composition: Oceanic crust is primarily composed of basalt and gabbro, which are mafic rocks rich in magnesium and iron. Continental crust is mainly composed of granite and other felsic rocks, which are richer in silica and aluminum.

* Minerals: The minerals in oceanic crust are denser than those in continental crust. For example, basalt contains minerals like olivine and pyroxene, which are denser than the quartz and feldspar found in granite.

* Thickness: Oceanic crust is thinner than continental crust (about 5-10 km compared to 30-70 km). This thinner nature contributes to its higher density.

The density difference is a key factor in plate tectonics. Denser oceanic crust subducts (dives under) less dense continental crust at convergent plate boundaries. This process drives volcanic activity and mountain building.