Here's why:

* Acceleration due to gravity (g) is constant near the Earth's surface. This means that all objects, regardless of their mass, experience the same acceleration downwards due to gravity.

* The force of gravity depends on mass. A more massive object will experience a greater force of gravity.

* Newton's Second Law (F = ma) explains the relationship. The force of gravity (F) acting on an object equals its mass (m) times its acceleration (a). Since the force of gravity increases with mass, but the acceleration due to gravity is constant, the mass cancels out.

In simpler terms: While a heavier object experiences a stronger pull of gravity, it also has more inertia, meaning it's harder to get it moving. These two factors balance each other out, resulting in the same acceleration for all objects near the Earth's surface.

Important Note: This is only true near the Earth's surface. As you move further away from the Earth, the acceleration due to gravity decreases, and the mass of the object becomes more relevant.