The oceanic crust sub ducts because it is thinner and denser than the continental crust at the convergent boundary where the convection current pulls crustal material down.
The theory is that convections currents in the mantle provide the force that moves tectonic crustal plates. At divergent boundaries like the mid atlantic ridge the convention currents are moving upward pushing new crustal material outward. At convergent boundaries like a subduction zone. the convention currents are moving downward pulling old crustal material back into the mantle.
At a convergent boundary like a subduction zone crustal material is moving down, as the crustal plates come together. The heavier (denser) oceanic crust made mainly of basaltic rock is pushed under the lighter continental crust made mainly of granite. The Thicker continental crust rides over the top of the thinner oceanic crust, as the oceanic crust is drawn down into the mantle and melted.
This raises a question as to how thick layers of ocean sediments are found on the Continental crust relatively undeformed. Most ocean sediments observed today are being destroyed at the subduction zones.