Why does oceanic crust slide under continental crust during subduction?

The oceanic crust slides under the continental crust due to the differences in their densities. The continental crust is more felsic (contains more silica) which makes it lighter than the oceanic crust which is more mafic (containes more fe and mg).

Because the process of subduction is very slow, gravitational forces have a stronger effect on the more dense oceanic crust, causing this crust to be pulled under the continental crust and down into the mantle.