The change from multi-racial indentured servitude to race-based, perpetual slavery came about largely for three main reasons. One reason had to do with race while the others had to do with the availability of white indentured servants and the results of having freed servants in the colonies.
Early on in the Chesapeake region, the majority of unfree labor was provided by indentured servants. Most of these indentured servants were white. It is said, for example, that planters in the Chesapeake had brought roughly 100,000 white indentured servants to that region by 1700. Beginning in 1619, some African slaves and indentured servants were brought to the American colonies. However, for most of that century, black slaves were relatively few (perhaps about 7% of the population of the southern colonies in 1670).
But then, the supply of white indentured servants dried up. Economic conditions in England improved, making it so that many fewer whites were willing to indenture themselves to get to the colonies. At the same time, many plantation owners were coming to worry about the ramifications of having large numbers of ex-indentured servants living in the colonies. They were afraid that the freed servants would come to be so numerous that they would take over more control of the colonies and would act in ways that would hurt the planters.
At the same time, planters were motivated to enslave Africans, at least in part, on the basis of race. They worried that large numbers of Africans (if they were freed) would be even more dangerous than freed white servants. They also felt that they could justify perpetual slavery with regard to Africans because of the Africans’ supposed racial inferiority.
Over time, then, laws in the colonies came to single out Africans as a distinct population whose default status was that of slaves in perpetuity. This development did not happen overnight, but when it did happen, it changed the face of the United States forever.