Personally, I chalk up most of the failure to utilize Amerindian populations as mass slave labor to the profound demographic collapse of native populations. There was considerable mobilization of the Indian populace of Meso-America for instance - In the first generation after conquest ( when the Spanish were virtually parasitic on Aztec culture ). The problem is they all died out.
Pre-European population figures in the Americas is a controversial topic. I’ve seen figures in Meso-America ( by far the most densely populated region ) ranging anywhere from 5 million to 30 million. But it appears the consensus is moving towards the larger figures. Tenochitlan alone may have boasted a population of over 250,000. If we take a high average of perhaps ~20 million as a starting point in 1519 ( the year of Cortes’ conquest ), we see a decline to about ~2.65 million in 1568 ( based on Spanish census data ), then a further decline to ~1.1 million in 1605.
We see similar patterns elsewhere. The Inca State for example is estimated to have contained somewhere in the range of 7 - 11 million subjects. If we start with the low average of 7 million in 1532 ( Probably appropriate since the North had already had been hit by smallpox traveling in advance of the Spanish overland from middle America, down through the isthmus of Panama, and into modern Ecuador. There it prematurely killed the highly successful Inca and recent conquerer of that region, Manca Capac, precipitating the civil struggle for succesion between Atahualpa and Huascar that Pizarro was able to turn to his advantage so effectively ) we see a decline to ~2.5 million by 1560 and then to ~1.4 million in 1590. I understand the collapse on Hispaniola among the Arawak was even more startling. ( Interesting to note that the Andes almost certainly ended up with a larger native population by the opening of the 17th century than Mexico, despite starting from a base that was probably 1/2 to 2/3 smaller at the opening of the 16th. I have an UTTERLY FASCINATING ( ) hypothesis as to why this was so and the impact it had on native/Spanish relations in the two regions. But since I’d hate to bore you all to death, I’ll resist the urge to take start digressing ).
Similarly Native Americans were co-opted as slaves in North America. After the Tuscarora were defeated ( North Carolina? ), part were exterminated, part were taken as slaves ( I seem to recall a figure of 1,000 ), and a remnant retreated northward to join the Iroquois Confederacy as the ‘Sixth Nation’. But this was early, before the advent of mass plantation slavery ( although there was apparently some use of slave labor from Virginia on South growing tobacco, indigo, and rice ). By the time cotton began making slavery in its American form particularly economical, the easily accessible coastal populations were either completely gone or terribly decimated. No more Powhatan, Catawba, etc. . And population densities were likely never high ( outside of perhaps part of the Mississippi valley ) in NA anyway. Plus there was always the “retreat into the wilderness” option alluded to by other posters.
As to why black Africa were the preferred source - My guess was that it was simply the best and cheapest source by the 18th and 19th century. White Europeans had all been Christianized - eliminating the old East European reservoir that was so important in the Middle Ages. The Near East was dominated by the Ottomans and other decaying ( but still viable ) states that didn’t sell their citizens and weren’t worth the expense of raiding considering the numbers needed. Asia was either scarcely or imperfectly penetrated and the cultures there were not in the habit of selling their citizens either - They were worth more as labor to generate land tax. But in Africa you had multifarious tribes that were perfectly content to raid others en masse to sell as assets to European traders. Africa was the only really good pool for the tremendous numbers required.
Just as an aside - The Arabs did indeed make use of Black African slaves as mass agricultural labor. In the Middle Ages . Hence the devastating Zanj ( derived from Zanzibar ) Revolt of 869 - 883 that started in Basra and engulfed much of Southern Iraq and Southwestern Iran. One of many events that sapped the resources of the Middle Abbasids and led to the decline of central authority in the Caliphate. Remember that Mesopotamia was for millenia the agricultural heartland of the Middle East, well suited to the use of mass slave labor as an agricultural force. It was in fact the economic linchpin of not only the Abbasid Caliphate, but every state that had preceded them in the region back to the Achaemenids. It’s decline, brought on by centuries of over-irrigation ( with the concomittant rise in water tables and resultant soil salinization ) among other factors, was yet ANOTHER nail in the Abbasid coffin. Both in terms of actual income ( 100 million dirhams in land revenue in 800, 31 million in 919 ), but also in terms of the economic remedy, which was the iqta system of tax-farming. Though efficient in the short-term at raising cash and cutting expenses, this ‘Islamic Feudalism’ further diluted the central authority of the Abbasid state over time.
Oops - There I go digressing again. Sorry .