There were several key factors that contributed to the expansion of the Atlantic slave trade, but the most influential was likely the development of plantation agriculture, especially sugar plantations, in the Americas during the 16th-18th centuries. Some key reasons why the growth of plantations drove the expansion of the slave trade:
- Labor intensive crops like sugar, tobacco, coffee required large amounts of manual labor to be profitable. European indentured servants died quickly in the tropics, so African slaves were imported in huge numbers as a more reliable source of labor.
- The European demand for sugar and other commodities led to more land being cultivated in the Americas, requiring more slave labor. Sugar production especially skyrocketed in Brazil, the Caribbean, and American South.
- Slave labor was extremely cheap and profitable for the European colonists and plantation owners. The transatlantic slave trade became a major business enterprise and lucrative source of wealth for European traders and nations.
- Technological innovations in shipbuilding, navigation, and banking/finance enabled the transportation and sale of slaves in large volumes across the Atlantic. This drastically reduced costs and increased profitability of the slave trade.
So while African participation was essential at the onset, the massive expansion of plantation agriculture for European markets was the most significant factor that transformed the slave trade into a vast, transoceanic enterprise during the colonial era. The demand for cheap slave labor to cultivate cash crops like sugar drove the exponential growth of the trade.