Why and how did slavery in the Atlantic world (North America) become associated with people from Africa in the period from 1400 to 1800? Why was it not others?

Soon after Columbus landed in Dominica in 1492, indigenous peoples, probably members of the Carib tribe from which the Caribbean gets its name, were enslaved by European explorers turned conquerors. In 1502, the first African slave was brought to the island of Hispaniola, the island which now comprises Haiti on the west coast and the Dominican Republic to the east.
Native Americans did not last as slaves for several reasons. Firstly, many died as a result of exposure to European diseases, such as influenza, smallpox, and yellow fever. Those who survived resisted enslavement. Some were probably killed in confrontations—no match for the firearms that explorers brought—while others fled into the jungles they knew very well.
The British colonies in the Caribbean and in the original thirteen colonies originally employed white indentured servants to work on plantations that produced cash crops, such as sugarcane and tobacco. However, this business model could not last. White workers, who exchanged a number of years of service in exchange for land, sometimes balked at the work, especially on sugar plantations. Also, more people farming land created more competition for planters.
The solution to import slaves from West Africa addressed the potential loss in profits, as well as concerns over losing labor forces. Africans would be less likely to run away, whites assumed, in foreign territory. They would also not be guaranteed land.
Europeans, particularly the Portuguese, who were very involved in trade in the 1500s, were already engaged with some African tribes and, with cooperation from kings and chieftains, traded people for gold or firearms. However, not all African tribes consented to the enslavement of their people, and some resisted attempts by Europeans to force conditions. Queen Nzinga, who ruled a territory that is now Angola and part of the Congo, successfully resisted an attempt by the Portuguese to enslave her people without her consent.
Africans were chosen out of convenience, and the belief that they would be better able to handle the heat of the Caribbean and Southern colonies. Though the strict color line that established blacks as "slaves" and whites as "free citizens" would not come into existence until the 1690s in the American colonies, Europeans were inclined to view blacks as inferior due to their status as heathens. This also applied to the enslavement of indigenous peoples. Enslaving those who had darker skins, unfamiliar customs, and non-Christian religious rituals was easier than forcing labor from fellow European immigrants—despite their lower class backgrounds.