Why Ocean floor always goes under continent?

At subduction zones, the line where continent and sea-floor meet, former always goes under the latter. Reason for this is rock contained in continental-crust is lighter than the rocks made by oceanic-crush.

In general, as we go down density of rock increases
Density of continental crust - 2300 kg/m3
Density of oceanic crust - 2700 kg/m3
Density of Mantle - 3300 kg/m3