Why will a 15 pound bowling ball and a 3 pound horseshoe dropped from the same height both hit the ground at the same time?

Because while a larger mass has a proportionally greater inertia (resistance to movement) than a smaller one, the pull of gravity also affects it that much more. The net result is to exactly cancel out the forces, allowing all objects within a given gravitational field (regardless of their mass) to fall at the same rate... unless there's another force, like air resistance on a feather, to tip the balance.