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

The traders' greed led them to cram more slaves on the ship than they had room for, leave them sitting in filth and stench, and caused them to provide to little food. All of these things caused the death of many onboard.

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.

The shrieks of the women, and the groans of the dying, rendered the whole a scene of horror almost inconceivable.