I can't pretend that the intimate details of Native American forced labour are something that I'm versed in nearly as much as other aspects of the development of slavery, but I can offer you a broader perspective through a comparison with African slavery.
Something to be aware of is that North American colonies did not jump suddenly and enthusiastically into the racial system of slavery that ultimately came to be such a defining feature of North American society; there were very meaningful, large-scale experiments with both indigenous enslavement and with white indentured labour ( the conditions of which sometimes proved to be comparable to slavery). In the case of Native American slavery or servitude, there's still some debate and discussion about how widespread it was and how authentically comparable it was in either social reality or ideological construction to African slavery, and the scholarship isn't nearly as well developed. In particular, our understanding of Native American enslavement is complicated by the fact the racial paradigm of New World slavery was still emerging in the colonial period; it has been argued quite convincingly that a lot of the Native American experience has been obscured because they did not fit easily into this emerging paradigm and could alternatively end up being lost in official records as black people, "people of colour", or even in some specific instances as Europeans.
As for why this system of slavery or proto-slavery failed to take hold in the same way that transatlantic African slavery did, the susceptibility of the native populations of North America to European disease is one of the more well known explanations offered. It is not, however, the full picture. In trying to construct a system of forced indigenous labour, white European colonists were not working in a vacuum; native American societies already had their own conceptions of what slavery meant and implied, and their own history of both enslavement and resistance to it. The colonists were attempting to create a malleable labour force in their own land of origin, which made quelling resistance extremely difficult (EDIT: and resistance, it occurs to me I should emphasise here, is much more complicated than just violent resistance or rebellion). Enslaving an indigenous person meant effectively physically wresting them from their own community and then trying to coerce them into servitude to cultural aliens on the threat of violence or degradation alone, whilst keeping them in a geographical context that often made escape or evasion a very real, very viable option. If you enslaved several indigenous people from the same locality, then they had likely had or could quickly develop a certain level of mutual familiarity even if they weren't from the same society, which makes it difficult to create a submissive workforce dependent upon their master.
In contrast, everything about African slavery was constructed to create a highly malleable, obedient workforce. African slaves were completely removed from their homeland and transported to an alien land they had no understanding of. Being drawn from very wide parts of their continent and belonging to very diverse societies, any given slave ship or plantation would have been home to a wide range of languages, religious beliefs and cultural norms. The physical experience of being enslaved, held in captivity and then transported across the Atlantic was a devastating and dehumanising one that left no illusions about what their worth and intended purpose was. For meaningful resistance to be possible, African slaves had to first construct entirely new senses of identity and community for themselves, which would inevitably be a slow process and have to be rooted in the one common experience they all shared: enslavement. It also required an ability, willingness and strength of character to overcome the extreme physical abuse and psychological degradation inherent to racial slavery. And unlike Native Americans, some of who perhaps may have had hope of being able to find their way back to their original community or to another society that would take them in as a kind of refugee, there was no feasible possibility of escape for most Africans.
For plantation economies with a thirst for easily controlled, dirt cheap, hardy physical labour, African slaves were always likely to end up as the preference of the planter class in the long-term, particularly given the emerging racial paradigm. The construction of the African slavetrade created a vast, highly malleable and socially vulnerable work-force in a way that white indenture or indigenous enslavement simply could not.
EDIT: If you're reading this comment scroll down to see the other two as well, and especially /u/anthropology_nerd's, which elaborate much better on specifics than I can here.
As others have already mentioned, Native Americans were the go-to choice for cheap exploitable labor for the first few centuries of contact. The popular history of the Americas grossly underestimates the role of both small and large scale slaving raids on the demographic response to contact, as well as the role of slaving as one of the first tendrils of contact spreading in advance of European arrival. Slaving raids, combined with warfare, epidemic disease spread, territorial displacement, and other factors worked in combination to decrease Native American population size after contact, and all of these factors limited the possibility of a quick population recovery. Native American slavery existed before contact, but fueled by factors ranging from self-preservation to increasing access to trade goods, the trade transformed into something not yet seen in the Americas.
Allow me to quote from an earlier answer...
Before first contact with officially sanctioned entradas to Florida in the early 1500s, unofficial traders and fisherman plied the Atlantic and Gulf Coast. These unofficial voyagers routinely augmented their stores with unwary captives, either for sale as slaves in the Caribbean or Europe, or to serve as translators for later voyages. During the first official entrada to Florida in 1513, the Spanish encountered Native American populations along the coast that already understood a few words of Spanish, and fled from the new arrivals, leading Juan Ponce de León to assume slaving raids preceded his arrival. When Verrazzano explored the Atlantic Coast in 1524 he encountered coastal populations who refused to trade directly with Europeans, preferring instead to exchange goods boat to boat across a line, possibly in an attempt to keep their distance and prevent abduction. The famous Pautuxet, Tisquantum/Squanto, was likely subject to several of these small-scale abductions along the coast before the arrival of The Mayflower.
Abductees were routinely sold in either the Caribbean or Europe, or trained as translators for future conquests. Accepted Spanish policy for new entradas included an initial journey to abduct a few young men, train them as translators, and return a few years to conquest and establish missions. This method was used to great success in Peru, when Pizarro captured two young boys from the coast in 1528. One of the young men, Martinillo, served as a translator during the famous showdown in Cajamarca in 1532 that resulted in the capture of Atahuallpa. In another example, Don Luis, a young man abducted by Spanish missionaries in 1561 from the Virginia tidewater region, returned in 1571 with a party of Jesuit fathers hoping to establish a mission near the James River. He escaped, organized the martyrdom of the Jesuits, and later advised Wahunsenacawh/Powhatan to expand the Powhatan Paramount Chiefdom to oppose Spanish encroachment. Wahunsenacawh/Powhatan’s daughter, Matoaka/Pocahontas, would later travel to England in 1616 where she was presented to the King James and Queen Anne as the daughter of “the most powerful prince of the Powhatan Empire of Virginia.”
Large scale slaving, and slaving raids, became a tool of war for English once they began to establish permanent settlements in the New World. The peace established between Plymouth and the Wampanoag lasted a generation. Massasoit’s son, Metacomet/Phillip, succeeded his father as sachem and due to a variety of factors organized the hostilities now known as King Phillip’s War. When the dust settled more than 3,000 Native Americans were killed and hundreds of survivors who were not professing Christians were sold into slavery in Bermuda.
The Carolinas used slaving raids as a tool of war against Spanish Florida, as well as a means of raising capital. Traders employed Native American allies, like the Savannah, to raid their neighbors for sale, and groups like the Kussoe who refused to raid were ruthlessly attacked. When the Westo, previously English allies who raided extensively for slaves, outlived their usefulness they were likewise enslaved. As English influence grew the choice of slave raid or be slaved extended raiding parties west across the Appalachians, and onto the Spanish mission doorsteps. Slavery became a tool of war, and the English attempts to rout the Spanish from Florida included enslaving their allied mission populations. Slaving raids nearly depopulated the Florida peninsula as refugees fled south in hopes of finding safe haven on ships bound for Spanish-controlled Cuba (a good slave raiding map). Gallay, in Indian Slave Trade: The Rise of the English Empire in the American South, 1670-1717, writes the drive to control Indian labor extended to every nook and cranny of the South, from Arkansas to the Carolinas and south to the Florida Keys in the period 1670-1715. More Indians were exported through Charles Town than Africans were imported during this period.
In both acts of small-scale abduction, as well as organized large-scale slaving raids, slaving often served as the first shock of contact between coastal Native American populations and European arrivals. The repercussions of slaving raids spread far in advance of European settlers, shattering previous lifeways, and sparking the rise of powerful confederacies like the Creek, Choctaw, Chickasaw, and Cherokee to combat slaving raids.
For more information check out Gallay's work, as well as Mapping the Mississippian Shatterzone: The Colonial Indian Slave Trade and Regional Instability in the American South, and Rushforth's Bonds of Alliance: Indigenous and African Slaveries in New France.
Just as a matter of correction, there were a fairly large number of Native people in North America who were bought and sold as slaves. According to Alan Gallay, some 30,000 to 50,000 indigenous slaves were exported from the port of Charleston, SC from the late 17th century to the early 18th century.
In point of fact, there were more Native slaves who were exported from Charleston (mostly to the Caribbean) during this period than there were African slaves who were imported.
The slave trade at Charleston was in turn an outgrowth of indigenous warfare that was both exploited and encouraged by the British colonists - in particular, South Carolina backed their Native allies' raids into Spanish Florida, where they captured and enslaved people in missionary villages.
The Native slave trade declined after the Yamasee War of 1715, however, in large part because the colonists realized that slaving expeditions could have a deleterious effect on their relations with neighboring indigenous groups. Because the purchase of African slaves did not have any diplomatic consequences in America, it seemed like a much safer choice.
I heard in my European school that natives were enslaved at the beginning but some priest saw it and brought two slaves to the Pope in Rome. The Pope saw that these people were not different from European people and thus could be declared human. As human could not be enslaved, they started enslaving Africans. Is any of this true?
