There were a number of reasons for this. First, African slaves were relatively inexpensive to purchase. The transatlantic slave trade was a well-established and efficient system, and the price of slaves was relatively low compared to the cost of indentured servants or the risk of using Native American labor.
Second, African slaves were seen as being more compliant and更容易控制than other types of labor. They were often dehumanized and treated as property, which made it easier for slave owners to exert control over them. Indentured servants, on the other hand, were often skilled laborers who had entered into a contract with their master for a specific period of time. This gave them some leverage and could make them more difficult to control.
Finally, African slaves were seen as being more resistant to disease than either Native Americans or indentured servants. This was especially important in the early days of European settlement in the Americas, when disease posed a serious threat to the survival of both colonists and their laborers.
For these reasons, African slaves became the primary source of labor in the Americas, and they played a central role in the development of the region's economy and society.