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 rich in silica and aluminum.

* Density: Mafic rocks are denser than felsic rocks. This is because the heavier elements like iron and magnesium are more abundant in mafic rocks.

In summary: Oceanic crust, being denser, sinks beneath continental crust at convergent plate boundaries, leading to subduction zones. This process is responsible for the formation of mountains, volcanic arcs, and earthquakes.