Compared to North America, in South America and the Caribbean, the gold and silver repositories were too scarce to provide a steady supply. For this reason, Europeans had to devise a new plan and turn to farmlands for profit. Soon enough, the colonizers realized that they could not expect high yields if they only depended on native populations for slave labor.
Besides, their numbers were depleting because Europeans brought a whole host of new diseases against which indigenous people had no immunity. The conquerors could not hire their own people, who would demand fair pay as well as legal protection from their employers.
Therefore, it was West Africans who were chosen to be forcefully displaced and brought to the colonies. In the United States, the birth rate among slaves was high enough to beat the mortality rate and reproduce the population. Since the slave status was hereditary, the owners were confident about always having a workforce at their disposal.