Here's why:

* Inertia is a fundamental property of matter. It's the tendency of an object to resist changes in its motion. It depends solely on the object's mass.

* Velocity is the rate of change of an object's position. It describes how fast and in what direction an object is moving.

In simpler terms: Inertia is like the object's "laziness." A heavier object is "lazier" and harder to get moving or to stop once it's moving. Doubling the velocity doesn't change the object's "laziness," just how fast it's moving.