Why does oceanic crust sink beneath continental crust at convergent boundaries?

Oceanic crust is denser than continental crust.

Explanation:

The density difference arises because of partial melting.

When you partially melt a rock, the lighter minerals melt before the denser ones, so the rocks that form from the melt are less dense than before.

After each cycle of partial melting, the rocks become progressively less dense.

The continental crust is formed more cycles of partial melting than oceanic crusts. As a result, the density of continental crust (2.7 g/cm³) is less than that of oceanic crust (3.3 g/cm³).

Consequently, the denser oceanic crust is subducted below the continental crust at a convergent boundary.