Horseback archers steady themselves using their legs, keeps the bow from bouncing around while aiming, and it's pretty much all in the legs too since they are leagues better than using a spine. Centaurs don't have that as their legs are the horse, the spine can't compensate the force of running, which involves the up and down motion that can easily throw off the aim. That's the cliff notes version, you can probably find others that talk more of it in better detail. As an alternative, centaurs would be phenomenal with a longer weapon with some weight behind it, like a halberd, they charge and swing that at you it would be a devastating blow, since it would be the speed of a horse, the twist of the waist and the weight of the weapon. Anyways, sorry to go on so long.