The slave trade was part of a profitable "triangular" trade in which the English shipped finished goods to Africa, picked up loads of slaves, transported the slaves to the United States, and then carried raw materials back to England. The basis for many of the conditions on the slave ships was an economic one, similar in a far more gruesome way to what passengers flying in economy class now experience. Simply put, the more bodies that can be transported, the more money the people doing the transporting make. Since slaves had no choice about whether to be on the ships, there was a simple profit calculation that so long as the attrition rate by death reduced profits less than carrying fewer slaves would, there was no economic purpose to improving conditions.
Using heavy chains to control slaves reduced the number of crew members needed to control slaves and mistreatment was thought to break the slaves' spirit and make them more tractable. Race came into play because the slaves were regarded as "other" and therefore less than fully human, and thus mistreating them was not considered morally or legally wrong by many of the people in the period. Only gradually did the Abolition movement succeed in changing these views.