Question

Why do two objects of different mass and dropped from the same height hit the ground at the same time?

Answer 1

The acceleration rate "a" is the gravitational Weight force (M g) divided by mass (M). That is Newton's second law. But weight is proportional to mass, so the mass cancels out.
acceleration = Mg/M = g

When the effect of air resistance is included, objects that are less dense or smaller tend to fall more slowly. A billiard ball will fall to the ground sooner than a tennis ball of the same size. Only in a vacuum do all objects fall at exactly the same rate.