Why do objects on earth accelerate downward at the same speed regardless of their mass?
What you mean here is that they accelerate downward at the same rate (“speed” has a particular meaning that isn’t so well suited to discussions of acceleration). This fact comes about because, although massive objects are harder to accelerate, they also experience more weight. Thus a huge stone will fall at the same rate as a small rock because the stone will be pulled downward more strongly by gravity and that extra pull will make up for the stone’s greater inertia.