Answer:
Because the rock that make up the oceanic tectonic plate is denser than the rock make up the continental tectonic plate.
Step-by-step explanation:
Subduction is the process when to tectonic plates were together, crashing one with the other. Normally is one oceanic tectonic plate with a continental tectonic plate. The reason of this, is that the first one has a lithosphere part that is denser than the second one That means that is harder, and is easier to go attract to the nucleus of magma of our planet.
When this process happens, since the harder rock goes to the mantle, gives a result that a the magma that is in this area, rises and erupts as in a volcano in the continental tectonic plate.
Hope this info is useful