Oceanic crust slides under continental crust during subduction because oceanic crust is

Oceanic crust slides under continental crust during subduction because oceanic crust is denser.

Explanation:

In a convergence boundary where continental crust collides with an oceanic crust , the oceanic crust get subducted due to it more dense nature. The oceanic crust is denser because of the kind of rocks found in it. The oceanic crust is dominated by mafic rocks(basalt, Gabbro and Diabase) majorly,  Basalt. Generally, this kind of rocks are heavier than the rocks found in the continental crust(usually granitic) . This kind of rocks (mafic)contribute to it(oceanic crust) dense nature.  

Due to the dense nature of oceanic crust, when they collides with the more active continental crust they tend to subduct. The subduction zone causes an introduction of magma as aesthenosphere material are released through this zone . This zone is marked with volcanoes . The oceanic crust subduct to form trenches.

The picture above illustrate the colliding effects of a continental  and oceanic crust.