Here's why:

* Denudation is the general term for the processes that wear down and remove material from the Earth's surface. This includes erosion (wind, water, ice), weathering (breaking down rocks), and mass wasting (gravity-driven movement of soil and rock).

* When erosion specifically removes material from the tops of mountains, leading to a decrease in their mass, it's a key component of denudation.

So, while erosion is the specific mechanism, denudation is the broader term that encompasses the overall process of lowering and reshaping the Earth's surface.