Because it would bring uncomfortable questions about the list of states that employ slavery, or effective slavery :
Sudan, Mali, Niger, Chad, Saudi Arabia, Qatar, Kuwait, Pakistan, Bangladesh.
Hmmm I wonder what is driving the advancement of slavery, given that's the places where it's happening. An exploration of slavery would bring the very uncomfortable, but pretty well known, fact to the surface that slavery is part of a particular religion. And the fact that if that religion entrenches itself in a society, it has a very strong tendency to bring slavery back.
(Fun fact : know what the arabic word for slave is ? "Abed". Literally it means "black". "Black gold", when it was a popular term in the 19th and 20th centuries, referred to slaves, not oil.
Others have discussed the slavery claims, I'll address the linguistic claims.
The three-letter root عبد, transliterated "3abd", or more inaccurately, "Abed", has meanings related to servant and slaves. It does not mean black. This can be seen in this entry of the Hans Wehr dictionary - http://tinyurl.com/pk2alcu . You might be familiar with this root from the Arabic male name Abdallah or 3abdallah - this literally means "Servant of God".
Side note - I'm using the letter 3 to indicate the http://en.wikipedia.org/wiki/Ayin letter, which is a throaty consonant in Arabic that is hard for Westerners to pronounce and hear. Because of this, it is sometimes sliced off and ignored when Arabic words are transliterated to the Latin alphabet. However, making claims about the Arabic language while ignoring it is a bit like making claims about the etymology of English words while ignoring the difference between l and r - it can lead you seriously astray.
The color black is أسود / aswad (masculine) or سوداء / sauda2 (feminine) in Arabic, all derived from the three-letter root سود (s-u-d). Dictionary entry at http://tinyurl.com/q7cadg4 . You may recognize this root from the country سودان / Sudan. However, it should not be confused with Saudi Arabia, which is سعودية or sa3udia. Again, the Ayin is present in the word and gives it a very different meaning.
In general, Arabic has a number of consonants that are hard to distinguish for Westerners - this includes the Ayin, the gh (guttural r), the difference between aspirated and non-aspirated h, the difference between emphatic and non-emphatic t, s, d and dh. This means it is hard to represent Arabic accurately with the Latin alphabet without extending it in ways unfamiliar to regular readers. I tend to trust claims about Arabic if the Arabic alphabet is used to explain them, otherwise I would be fairly skeptical.
Your reply is technically correct, and yet also entirely false. It's of course true that the color is aswad. I was not talking about the color black being abd. It's just that in English people use the same word for black (the color) and black (the skin).
If it makes things more clear, abed means n-gger. Literally and figuratively.
I'm more inclined to believe that the reason is socio-economic rather than religious. Many of those countries you listed are also a source of cheap goods for the West.
Edit: just to clarify, what I'm saying is that the majority of adherents to any religion aren't fundamentalists. They choose which parts of their holy scriptures to follow, and ignore the rest -- the vast majority aren't even aware of all of them.
Human nature being what it is, they are more likely to follow portions of scripture that benefit them in other ways, e.g. economically. They may likely have chosen to behave the same way regardless of their religion; the religion merely provides a convenient excuse.
I don't dispute that there are economic reasons behind it. I'm not saying that enslaving people is done without reason in islamic countries.
The problem is those same economic reasons exist in the west. Yet slavery is not practised here. Why not ? Simple : the population would never allow it (not even in dictatorships). Yet the population in the middle east does allow it (or at least, they won't fight to abolish it). What is the difference ?
Note that the origin of difference goes back all the way to the later days of the Roman Empire. What changed was not that the middle east started to accept slavery as normal, what changed was that the west stopped accepting it. Why ? Well, simple : because of the rise of Christianity. Once slavery was gone, it re-appeared in the west twice (not counting slavery in, for example, Al-Andalus). Both times it was eradicated from within, and outlawed (after a while). Both times slavery reappeared because of trade contacts with muslim countries. In the middle east/asia/africa the reverse happened. Slavery was normal, the vast majority of the time, but every 300 years or so the slaves would successfully revolt, which generally lead to a decade-long abolishment of slavery (sometimes a once-off freeing of every slaves, sometimes 50 years of no slaves), but it would always return.
What is the difference ? I'm not claiming islam changes populations to accept slavery. Rather, I'm claiming Christianity changes populations to stop accepting slavery. And of course, I'm claiming that were Christianity/"post"-christian values lose sway, for example to islam, slavery returns.
This does not just apply to slavery, but equality in general as well.
Most of your other claims here have already been refuted, but here's one more. If "black gold" ever mostly meant slaves rather than oil, then the editors of the Oxford English Dictionary would like to hear from you. (They say: "colloq. (orig. N. Amer.) petroleum; mineral oil" and their earliest citation is from 1910.)
I just did a Google book search for "black gold" on 19th-century books, and checked the first 100 results. None of them had anything to do with slavery (or with mineral oil). So far as the Google n-gram tool's corpus knows, the term became popular around 1920-1940.
I have been able to find a few references to "black gold" meaning slaves, but so far as I can tell from the snippets available on the web they have nothing to do with Islam. In the few cases where I can tell who the slavers were, they were specifically Americans.
The English colonist in the New World imported white indentured workers at first, but found there weren't enough of them. The Indians in the Americas refused to work or proved to be poorly fitted for long hours of hard labor. The Europeans found it easier and cheaper to import Africans as slaves. By the seventeenth century, the African slave trade was booming in the Americas. The slave dealers made so much money from their human cargoes that soon Africans came to be known as "black gold".
> I have been able to find a few references to "black gold" meaning slaves
I am not claiming that no one has ever used the term to mean slaves.
What I am disputing is (1) your claim that in the 19th and 20th centuries the term was popular with this main meaning, and (2) your claim that it had anything to do with Islam.
On #1, I find that the term was extremely rare until it started being used to describe oil in the 20th century.
On #2, note that the slavers mentioned in your particular links were all Western European or American. Guess how many of them were Muslims? I'm thinking the answer is somewhere between "exactly zero" and "zero to within statistical sampling error".
Black gold used to mean the slave trade towards America and within the British Empire and the Dutch. What does that have to do with Islam : simple. Who did they buy the slaves from ? The Ottoman empire.
Most kidnapped slaves never did end up in the western slave trade by the way, but were instead kidnapped into north africa and the middle east (which was the effective center of the muslim empire, they even relocated mecca into africa). Almost none of them survived, and today with a few small exceptions no significant numbers of those slaves survive.
I am not an expert on the slave trade, but my understanding is that most of the slaves in the western slave trade were bought in West Africa, which was never part of the Ottoman Empire.
(I'm not saying that the Ottoman Empire was never involved in the slave trade. It was. So was the British Empire.)
Sudan, Mali, Niger, Chad, Saudi Arabia, Qatar, Kuwait, Pakistan, Bangladesh.
Hmmm I wonder what is driving the advancement of slavery, given that's the places where it's happening. An exploration of slavery would bring the very uncomfortable, but pretty well known, fact to the surface that slavery is part of a particular religion. And the fact that if that religion entrenches itself in a society, it has a very strong tendency to bring slavery back.
(Fun fact : know what the arabic word for slave is ? "Abed". Literally it means "black". "Black gold", when it was a popular term in the 19th and 20th centuries, referred to slaves, not oil.