Why did African slavery become one of the most basic consequences of the Age of Discovery?

The Age of Discovery, also referred to as the Age of Exploration, describes the time period beginning with the early fifteenth century and extending through the seventeenth century in which expanding European exploration signified invasion and colonization for many non-European peoples. Explorers during this time period primarily set off in search of new trade routes. In many cases, they also discovered lands that were unknown to Europeans but already inhabited by indigenous groups. Although slaves have been bought and sold to some extent since ancient times, the trend dramatically increased in the 1580s as millions of people were kidnapped from Africa and sent to the Americas to be sold as slaves. The Portuguese were the first Europeans to purchase Africans captured by rival tribes and send them to the New World to be sold; by 1600, many other European countries engaged in this quickly expanding transatlantic slave trade.
The need for plantation workers in the American colonies at this time was great because indigenous workers were susceptible to diseases brought by European settlers, and much of the indigenous workforce had died by the 1550s. The three-stage journey of many ships, also known as the triangle of trade, brought goods such as guns or cloth from Europe to Africa, exchanged European goods for African slaves, transported slaves to the Americas, and then sold human lives in exchange for crops unique to the Americas, transporting these crops to Europe on the return passage. The Age of Discovery signified greatly increased accessibility between continents, most notably between the Old World and New World. This trend toward globalization led to an increase in European colonization. The transatlantic slave trade was a direct result of European colonization and was considered necessary to ensure that the workforce in the New World was able to adequately supply goods such as cotton, sugarcane, tobacco, potatoes, maize, sugarcane, silver, and gold to Europe and the rest of the Old World.