It is because of subduction: oceanic crust tends to get colder and denser with age as it spreads off the mid-ocean ridges. It gets so dense, that it sinks in the upper mantle (subduction). This is like a giant recycling system for oceanic lithosphere! Because continental crust is lighter than oceanic crust, continental crust cannot subduct. We therefore still have some very old continental rocks at the surface of the Earth.