Why does Equiano blame the illness aboard the ship on the " improvident avarice" of the traders?

In their greed, the slavers did not insure the well-being of their captives.... they merely wished to move from one point to another, uncaring of the final result.

The closeness of the place, and the heat of the climate, added to the number in the ship, which was so crowded that each had scarcely room to turn himself, almost suffocated us. This produced copious perspirations, so that the air soon became unfit for respiration, from a variety of loathsome smells, and brought on a sickness among the slaves, of which many died, thus falling victims to the improvident avarice, as I may call it, of their purchasers. This wretched situation was again aggravated by the galling of the chains, now become insupportable; and the filth of the necessary tubs, into which the children often fell, and were almost suffocated.