The oceanic lithosphere is subducted primarily due to its density and the type of materials that make it up. Oceanic lithosphere is generally denser than continental lithosphere because it is composed mainly of basalt and other dense volcanic rocks. When oceanic and continental plates collide, the denser oceanic plate tends to sink or subduct beneath the less dense continental plate. This process of subduction creates deep ocean trenches and is responsible for many geological activities, such as earthquakes and volcanic eruptions.
In contrast, the continental lithosphere is relatively thick and less dense, primarily composed of lighter granitic rocks. This lower density means it does not easily sink into the mantle when it meets oceanic lithosphere. Instead, when these two types of lithosphere collide, they often result in the folding and uplifting of the continental crust, leading to the formation of mountain ranges rather than subduction.
Overall, the fundamental difference in composition and density between oceanic and continental lithosphere is the key reason why oceanic plates are subducted while continental plates are not.