Why did the United States use slaves instead of free indentured workers?

One of the reasons that colonial America began to turn increasingly towards slavery instead of indentured servitude was Bacon's Rebellion of 1676. In this popular uprising, a group of Virginians rebelled against the elite governor of the colony. In the uprising, slaves united with white indentured servants to demand the right to move westward to claim land where Native Americans lived.
As a result of this uprising, the elite of Virginia began to impose a harsher color line and to make slavery for kidnapped Africans different from indentured servitude for whites. Their intent was to break up the potential solidarity between the groups. In addition, southern planters needed labor for labor-intensive crops such as tobacco, sugar, and cotton, and they increasingly relied on slaves to harvest these crops.