How do you explain the evolution of indentured servitude to hereditary slavery?

As is often the case, economic reasons lie at the center of this answer. In this answer, we will examine the evolution of indentured servitude to hereditary slavery in the British North American colonies. We can trace this story back to the earliest British colonial settlement in Jamestown, Virginia. The settlers of the Jamestown colony, after some early struggles, discovered that the colony was profitable, particularly through the growth of cash crops like tobacco. As the Jamestown colony grew and profitability was realized, the size of the colony began to expand.
With a goal of gaining wealth in the "New World," colonists would acquire land and begin to farm it (for our example we'll say for tobacco). How can you grow more wealthy through this? Well, theres a high demand for tobacco, so if a colonist were to grow tobacco and sell it, they would earn money. Now if you can grow even more tobacco, you can earn even more money. The problem encountered by these early planter colonists was the lack of labor that they faced. In order to grow the crops, you need people to work the land, especially as the land you are farming is growing larger. The initial way that this lack of labor was addressed was in the form of indentured servitude.
The "New World" provided opportunity for Englishmen. Going to the "New World" meant leaving behind the struggles of England for the chance to possibly gain land and wealth in the American colonies. The problem for many adventurous and willing Englishmen was that passage by boat to the Americas was quite expensive, and thus only those with some considerable wealth were able to get to the colonies. Indentured servitude sought to solve the problems faced by the colonial landowning farmers who needed workers and those Englishmen who wanted to leave England for the colonies but didn't have the money.
Through indentured servitude, poor Englishmen who wanted to go to the American colonies would have their passage paid for by a wealthier individual. An agreement would then be made in which the indentured servant would pay back the person who funded their journey by working for them, usually for four to seven years. In addition, the indentured servant would receive a place to sleep and some other basic necessities. An indentured servant would have some rights, but not the same freedoms as somebody who was not an indentured servant. It also provided them with an opportunity to get a foothold in the Americas. When the indentured servant had completed their work agreement, they often were granted some land and were able to start working on creating their own wealth.
As the colonies continued to grow, the demand for indentured servants increased. When demand for labor increases, the price of that labor also increases; thus, indentured servants started to become more expensive.
Let's take a look at a quick hypothetical example to better understand this point. If I want to go to the Americas from England as an indentured servant but there are not a lot of landowners who are looking to bring over an indentured servant, my options are limited. Additionally, I am not the only one who wants to go as an indentured servant, so if my terms are not suitable to the landowner, then he can simply attempt to negotiate with a different indentured servant for a better rate, and I don't get passage to the Americas. As the colony grew, so did the number of landowners, and the demand for indentured servants grew with them. Now if I don't agree on the terms with one landowner, I can seek out a different one. Additionally, there is a growing number of landowners searching for indentured servants and only a limited amount of indentured servants, so if they want to have an indentured servant, they need to have a better offer available, which likely means a lower term for how long the servant will work for them.
So with the growing demand for indentured servants and thus the growing price, landowners sought to find a different solution in order to keep their profits high. Additionally, many of the wealthy landowners felt a sense of threat from indentured servants, as they sometimes gained considerable wealth and power themselves after their term as an indentured servant was finished. In the mid 1600s, some colonies began to pass slave laws. These slave laws began to codify a system where African slaves brought to labor in the colonies would not be able to gain freedom and would have very few, if any, rights. For wealthy landowners, it became cheaper and safer to purchase African slaves to work their land rather than pay indentured servants. They would also not have to worry about the slaves gaining freedom after a certain amount of time as they did with indentured servants. This meant greater profits for landowners, and thus it made business sense to stop the use of indentured servants and instead import enslaved Africans to increase their wealth.
I would also argue that there is racial superiority at play in the expansion of hereditary slavery in the Americas as well. Ethnocentric Europeans of the time saw themselves as the pinnacle of society. They saw Africans of the time, who had different beliefs and values, as being inferior, primitive, and savage. Many Europeans felt that African and Native American societies needed to be assisted in developing into societies mirroring those in Europe. Thus they would not feel much guilt towards the enslavement and treatment of Africans, because they did not view them as equals.
In conclusion, the increasing cost of indentured servants combined with the low costs associated with African slavery helped to phase out the system of indentured servitude. When you factor in the ethnocentric beliefs of Europeans and their views of African societies at the time, it is clear to see how indentured servitude would be forgotten for hereditary slavery.
http://www.pbs.org/opb/historydetectives/feature/indentured-servants-in-the-us/