Planters found it more profitable to work slaves to death than to sustain them.

Evidence

On average, half of the slaves who arrived from Africa died within a few years. Children also died at incredible rates, reaching nearly 50 percent on some plantations. Each year 5 to 6 percent of the slaves died, and the situation was worse during the frequent epidemics in the colony. Birthrates, meanwhile, hovered around 3 percent. Focused on short-term gain and for the most part unburdened by humanitarian concerns, many masters and managers in Saint-Domingue coldly calculated that working slaves as hard as possible while cutting expenses on food, clothing, and medical care was more profitable than managing them in such a way that their population would grow. They worked their slaves to death, and replaced them by purchasing new ones. — Dubois, Laurent, Avengers of the New World_ The Story of…, 148