Here's why:

* Seafloor Spreading: Ocean crust is constantly being created at mid-ocean ridges. As magma rises and cools, it forms new oceanic crust. This process pushes the older crust away from the ridge, like a conveyor belt.

* Subduction: The older oceanic crust eventually reaches a subduction zone, where it dives beneath the continental crust and is recycled back into the mantle.

* Limited Age: This process means that the oldest ocean crust is found farthest from the mid-ocean ridges, closer to the continents where it will eventually be subducted.

It's important to note that no ocean crust is truly ancient, as the oldest known oceanic crust is only about 200 million years old. This is because the process of subduction constantly recycles the ocean floor.