I'm not particularly qualified to discuss the Middle East and the Arab slave trade (it's not a comparative I've ever worked with and so my knowledge of the scholarship is too lacking) - but I would like to address your remarks about the United States and the transatlantic trade, because I think that you're operating under a few misunderstandings about the scope and scale of that trade, which in turn is making your point of comparison perhaps a little flawed.

You are correct in saying that less than a million Africans came to the United States on slave ships. An estimated minimum of 473,000 left Africa and some 389,000 arrived between 1628 and 1860. There are two very, very big caveats to those figures however. The first is that the trade to the United States represents only a tiny portion of the overall transatlantic slave trade; all in all, from 1501 to 1866, some 11million men, women and children survived the journey from Africa to the New World. Of these, 2.3million were trafficked to the British Caribbean - some of who were later sold on to the United States. Most however were taken to Portugese colonies in Brazil - an estimated 4.7million people, or an average of 15,000 people every year, arrived in Brazil from Africa during the course of the transatlantic slave trade. The Spanish made a good go of the slave trade, too, successfully transporting 1.3million souls to their colonies by 1866; the figures for the French Caribbean are around 1.1million by 1831.

So it should be apparent, then, that the United States - and the continental colonies that preceded them - are rather exceptional in how few slaves that they received directly from Africa in the transatlantic trade, despite a thriving domestic institution of slavery. And if we look to Brazil today, which was far and away the biggest recipient of slaves from the transatlantic trade, we find a Black Brazilian population group that constitutes 7.6% of the estimated total, compared to nearly 13% in the United States, excluding individuals of (identified) mixed heritage. Put in more striking terms, Brazil's black-identified population is roughly three times the number of African slaves brought there directly, whilst the United States' is well more than one-hundred times that number. So rather than asking how it is possible for the Middle East to have a relatively small diaspora of Africans despite a vibrant slave trade in contrast to the US, it would perhaps be better to switch your perspective and interrogate how it is the United States has such a large diaspora despite a small share of the trade. If your thesis about the numbers involved were correct (and /u/Commustar has addressed that in his post), it is in fact the USA, not the Middle East, which is remarkable.

Part of the answer to that question lies in the other misunderstanding I think you have about the dynamics of slavery in the United States and the role of the trade. Something that many people struggle to realise, particularly because of how slavery is represented in American popular culture, is that by the time of the USA's independence, the days of the transatlantic slave trade being essential to the growth of North American slavery had long since passed. The transatlantic trade was much more of a British enterprise than an American one; so much so that during the Revolutionary War, the future United States had suspended all of its participation in the transatlantic trade in a bid to undermine the British economy, future slave states included. It was never restored to its pre-war levels and banned in 1807. Yet according to the US census, whilst the slave population grew by roughly 245,000 in the last decade before the slave trade was finally killed at the federal level, it grew by almost 400,000 in the decade after abolition. Put another way: from 1790 to 1810, the slave population increased by an average of 22,000 souls a year. Yet from 1810 to 1830, it grew by 43,000. And if we look at that pre-1810 increase, we see most of it doesn't actually come from the transatlantic trade: the slave trade arrival estimates suggest only 20 - 25% of that 22,000 yearly increase can be accounted for in the form of newly arrived African slaves.

Instead, the remarkable thing about the United States was that it was developing its own internal, domestic slave trade - with states that had a perceived surplus of slaves feeding states that had a need for them. This was a trade that flowed in the post-independence period from the Upper South to the Lower South; and it was a trade that moved many, many more people than the transatlantic trade to the US ever did. States like Virginia, which had an abundance of slave labour relative to its economic needs, could sell their 'extra stock' down to a state like Georgia, where the constant expansion of cash crop farming creating an ever-growing demand for new labour. In particular, the invention of the cotton gin - which facilitated an explosion in cotton production from just 1.5million pounds in 1790 to 35million by 1800, up to an astounding 2.3billion by 1860 (see Bailey in Agricultural History 68:2, 1994) - provided the impetus for such a massive expansion of labour-intense farming in the Lower South.

By inference then we can establish something else remarkable and important in understanding why the United States has such a sizeable African American minority: it had a slave population that reproduced itself on a large and meaningful scale. All manner of factors contributed to this possibility - the United States did not have the same challenges with land availability for slave or free black communities as Barbados to work and inhabit independently, for instance. But the fundamental thing that made it possible was that slavery in the United States was not geared near exclusively towards high-intensity farming in the same way that the colonies of the British Caribbean were (though the picture of the Caribbean as a series of perpetual death camps is quite inaccurate; the slave population in the Caribbean did, in fact, reproduce organically, just not nearly as substantially), and the structure of slave life was more conducive to the forming of sexual and romantic partnerships and family units, particularly in the Upper South where farming operations were less intense. Even in the Lower South, the incentives were obvious for planters to try and facilitate natural population growth.

Remarkable, too, is the ideology that grows up around slavery in the United States - specifically, the ideology of race. In the US we see the emergence of an ideological framework that stands out in comparison to the rest of the world for two reasons: firstly, more than anywhere else in the world, the US feels the need to construct an aggressively pro-slavery ideology. For many southern slave-holders defending the institution post-independence, slavery was not a necessary evil or simply a benign institution - it was a good, thoroughly just and righteous institution that actually uplifted black people, who were held to be inherently lesser and in need of a firm hand to guide them. The South was not a society with slaves; it was a slave society. Which ties directly into the second point - the rigidity of race in the US.

Whilst questions of race are complex and nuanced wherever you go in the world, conceptions of race became - and still are - unusually rigid in the United States. I've just written a commentary here about the legacy of slavery on the racial dynamics of the British Caribbean that you might find interesting as a point of contrast, to see what you can detect in the way of similarities and differences. Very clear distinctions were created in the US between black and white, with all kinds of implications attached to them; this rigid barrier existed not only during slavery but in the institutions of segregation and anti-miscegenation that followed. Whereas Caribbean societies evolved to have highly complex, nuanced and contradictory ideas of what 'black' and 'white' implied - ideas that intersected with culture, language, religion, education, gender, sexuality and more - popular conceptions of racial hierarchy in the US arguably never became quite so fluid or nuanced. If a black woman had a child by a white man, the child was almost always seen as black in the US.

Thus the United States has historically been less able to facilitate interracial reproduction, and less willing to recognise - legally or culturally - the mixed heritage of children born to interracial families, particularly when they more obviously inherit the traits of a black parent. In contrast, the Caribbean has a rich history of having distinct mixed race classes and identities, as does South Africa (which implicitly recognised and legitimised such identities in its own very rigid racial framework during apartheid). And when we look to Brazil, we see that although the actual Black Brazilian population is quite small relative to the US, the percentage of people who identify as mixed race is much, much larger - more than 40% of the population I believe, of who a massive chunk attribute that mixed status to African heritage. So I suspect something similar happened with the descendants of African slaves from the Arab slave trade. In the absence of that rigid, racialised ideology, it stands to reason that it would have been significantly easier for their ethnic footprint to be lost through intermarriage, particularly given that manumission would have been much easier than in the USA.

In a nutshell then, it is probably the USA, not the Middle East, that is remarkable. Whilst I'm aware it doesn't do much to address your questions about the Arab trade, I do hope I've been able to shed some light on the comparative.

(Estimate figures from the Transatlantic Slavetrade Database)

First off, let's tackle the question of numbers of enslaved.

some have put the figure of blacks enslaved between 20 to 80 million

Who gives those numbers? Or where did you find them? In the wikipedia article that you cite, numbers between 8 and 17 million are used throughout.

Additionally, we need to also realize that not all people who were subjected to the Arab Slave Trade were destined to be sent to the Middle East. This table shows* that for the East African coast in the 19th century, about as many slaves stayed in East Africa as were sent anywhere else. Also of note is an estimated death toll of 9% for slaves sent overseas, which would mean even fewer Africans reaching the Middle East in the 19th century.

Also, I would ask how you define the Middle East? Are you including or excluding North Africa? For example, Egypt has a large population of Nubian origin, former President Anwar el-Sadat being one notable example.

*That table shows exports to the middle east as well as to South Africa and the Atlantic. Thus, it is not solely representing the Arab Slave trade but the Atlantic Slave trade as well.

This is absolutely a question in need of more research. The Wikipedia article offers only one citation on the African trade (fn 94 gives aggregate numbers, but it doesn't specify what was actually being totaled), whereas the other citations are only about defining geography without supporting the claims that people were being traded there.

That's not to say that the trans-Saharan trade didn't exist. The trans-Saharan slave trade was in continuous operation from antiquity until the 20th century. A high estimate for the years 1400-1900 is 14-15 million Africans. A more conservative estimate, but spanning the longer years 600-1500, is 5.5 million slaves. For further discussion, I'd recommend the superb overview by Anne Haour.

That's significantly less than the 20-80 million your sources propose, but the observation remains valid: How did less than a million black slaves generate such a prominent portion of the US population today, while 5-15 times that number seem to have had little effect on the demographics of the Middle East?

It's a hard question to answer, in part because modern definitions of slavery are so mixed up with notions of race. What makes it more complicated, however, is that the medieval texts that record interregional slave trade seem to adopt racial terms. A classic example is the Zanj rebellion, as cited by /u/rule2DoubleTap, which occurred in the 800s in modern Iraq. Although "Zanj" today is usually thought to mean black Africans and was possibly related to the word Zanzibar, it was used more flexibly during the middle ages.

It's pretty clear that the dominant part of the Zanj revolt included slaves or freedmen who could trace their families to East Africa. However, a close read of the sources also turns up people like a Byzantine slave named Qirtās, who was certainly not of African descent. (His name actually sounds Slavic to me, which would fit in well with what we know about the Eastern European slave trade at that time.) Moreover, it seems like the Zanj had a good number of peasants among their ranks. This fits in well with what we know about early Islamic society: manumission was frequent, and there was a lot of social mobility. For example, in the early 700s, Yazid ibn Abi Habib, the son of a Nubian captive, became Egypt's top legal authority.

That's probably where the answer to your question lies. Black slaves were not thought of as being essentially or existentially black slaves. They could and often were freed, and their free descendants could mix freely with the populations already living in the Middle East. In contrast, slavery was highly racialized in the US, and even after abolition, racial thought continued to determine how people perceived and lived in the world around them (as noted by /u/CommustarMo).

I would be interest about this also. My reading about the Caliphate during the times of Charlemagne through the first crusaders has indicated most of their foreign slaves came from Europe and the West.