Here's why:

* Moment of inertia is a measure of an object's resistance to rotational motion. It depends on the mass distribution of the object relative to its axis of rotation.

* Mass (M) is a fundamental quantity in physics, representing the amount of matter in an object.

* Length (L) is another fundamental quantity, representing distance.

The formula for moment of inertia of a point mass 'm' at a distance 'r' from the axis of rotation is:

I = mr²

This formula directly shows that moment of inertia is proportional to mass and the square of the distance. Hence, its dimension is ML².

Note: The unit of moment of inertia is typically kilogram meter squared (kg m²) in the SI system.