Here's why:

* Buoyant force is directly proportional to the volume of fluid displaced. The more fluid a submerged object displaces, the greater the buoyant force.

* Pressure increases with depth. The deeper an object is submerged, the greater the pressure of the surrounding fluid.

* Greater pressure means more force on the bottom surface. Since the pressure is higher at the bottom of the rock, the upward force acting on the bottom surface is greater than the upward force acting on the top surface.

This difference in pressure creates a net upward force, which is the buoyant force. Because the pressure is highest at the bottom, the buoyant force is greatest there.