Here's why:

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

* Thickness: Oceanic crust is thinner than continental crust. This means the weight of the overlying ocean water and sediments exerts more pressure on the oceanic crust, compressing it and making it denser.

This density difference is a key factor in plate tectonics. When oceanic and continental plates collide, the denser oceanic plate is forced to subduct (slide) beneath the lighter continental plate. This process is responsible for the formation of volcanic arcs and mountain ranges.