Traders on the slave ships and masters on the plantations showed lack of humanitarian concern, prejudice and racism towards the slaves. Their main concern was to make the land owners a profit.
If the slaves had more time on the ship's deck during the crossing, gaining exercise and fresh air, they arrived in better condition.
This was measured against the greater risk of revolt when slaves were unshackled, and the cost of employing sufficient crew to control unshackled slaves.