It seems largely the consensus today is that slavery was not really economically viable as a system. There's some dispute here, but that's the impression I've gotten.
Why, then, did slavery begin at all? Here I'm thinking particularly of slavery in the US which took root in the late 17th century. Was there a decision making process that led people to conclude that slavery would be a profitable enterprise? If so, what was this rationale?
I've read a bit about the Royal African Company, and a theory which has popped into my head that the king may have made some attempts to push slave labor for his own profit. But, my total knowledge here is really weak so I'm at a loss to push this theory further.
Since I am a United States historian, I will focus on answering the 17th century question in regards to places like Virginia. I will make a few references to Central and South America, but I will leave that to my more learned colleagues of that region to answer more fully.
This answer ended up being a long one so I will break it into multiple sections.
Also, one apology: due to quarantine we are severely restricted access to campus at the moment and a few of my references are in my office, so some further answers or clarifications may need to wait until I get access to some of my materials. (Such as I know I have an excellent book on early South Carolina Rice and Slavery and for the life of me I can't remember the title)
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First, when you speak of slavery, I am going to assume we will be focusing on black slavery, and not the abuse or using of labor of indigenous peoples. Again, I will leave my Latin American specialist colleagues to deal with a more specific history here, but the abuse of indigenous peoples was important early on, but as time went on the need for slave labor exploded, with Brazil being the largest importer of slaves in the Americas ultimately.
***Visual Helper: To get a visual impact of the trips and numbers of people that were brought over as slaves and where, Andrew Kahn, who was an interactive editor for Slate, did an animation based on peer reviewed work and it has been displayed at multiple museums and used in lectures called “The Atlantic Slave Trade in two Minutes.” Watch the voyages and the density represented by the size of the dots to get an idea of the balance of British North America versus Caribbean / Central / South American slave ship voyages. (https://www.andrewmkahn.com/interactives Andrew’s page and the Slate Article http://www.slate.com/articles/life/the_history_of_american_slavery/2015/06/animated_interactive_of_the_history_of_the_atlantic_slave_trade.html) While I am hesitant to cite a non-academic source, since it was compiled and has been corrected by academics (the graphic has been updated over the years with corrections) it will help put in context some of the comments that follow.
In the British North American colonies, there was not a guarantee that slave labor would become dominant – at least not at first. And indeed enslaved black labor was NOT as popular in the early 17th century for multiple reasons, but the primary ones involved the economics and mortality.
We know the first successful British colonial settlement of Jamestown also recorded some of the first enslaved peoples. But predominantly in British North America, it was indentured servitude that dominated labor, particularly in the South.
Colonial reasons for settlement also challenged the demographics of the types of labor that British North American Settlements entertained. To help get at why the economics and demographics of the south eventually came to embrace slavery, let me turn to historian Jack Greene. Although a bit reductionist and now considered a little oversimplified, Jack Greene’s Pursuits of Happiness (1988) is still a useful model (and incredibly well researched) for us to examine the 17th century lens of settlement. Greene suggested that (in general) the northern colonies were modeled after what he typified as the “Declension Model,” meaning they began primarily with religious concerns and eventually declined into more secular states of being after the decline of (particularly) the Puritan experiment. On the other hand, the South came for primarily economic reasons – what Greene typified as the “developmental model,” in that they developed along the lines of what England WANTED from the colonies economically. And again, in order to understand that we have to take a look at how British North American colonists understood economics, which was through the lens of MERCANTILISM.