Why were west african slaves seen as desirable for the slave system established in america? ( p. s short answer pl )

There was a confluence of elements involved. 

But, one thing that is routinely ignored is that a racial and religious hierarchy was entrenched in the Old Word, at the same time the Americas were colonized and the indigenous population was plummeting. Slavery was a historically accepted and prevalent institution in Europe, African and the Middle East (and elsewhere). But, what is commonly overlooked when speaking about the rise of African chattel slavery is the fact that this system had already been highly organized by Arab/Muslim society and then picked up by Europeans (adopting a racial component). They targeted sub-Saharan or Black Africans specifically for slavery and kept this practice going for many centuries. 

I’ll even let you in on a little secret that a lot of Arabs don’t readily admit. But, the common term for Blacks (meaning, sub-Saharan Africans) is ‘abeed, the same term for slave. It is synonymous, or interchangeable in completely normal conversation. This is not a fringe term used by racists nor is it seen as a particular course term. It is mainstream. It can even be found in some news articles and in the media. Arab Muslim countries were the last in the entire world to end this AFRICAN chattel slavery system. Even today the old system of chattel slavery is still practiced by Arab-identified slave owners in places like Mauritania and Sudan. 

There was a traditional religious component found in both Islam and Christianity, where Blacks were considered the descendants of Ham and had been cursed with Black skin and were justifiably natural slaves.  

So, as the Europeans started colonization of the New World, they met a people that did not fit into this old racial model. While there was some attempt to enslave American Indians initially, the practice never became entrenched (for a variety of reasons, including high mortality rates from lack of immunity to Old World diseases). However, what is perpetually downplayed in the racist component of African chattel slavery. Spain outlawed Indian slavery in 1530, and the Catholic church issued the Sublimus Dei which also clarified that Indians were free and could not be enslaved. The English and French also did not have the policy to enslave Indians based solely on their race. Most Indian slaves taken during the early colonial period were from “warring tribes” and often these slaves were sent to the Indies. There was no large class of Indians slaves. In what would become the USA, they represented very small numbers and enslavement of Indians was specifically outlawed by most colonies or states rather early in American history. However, European or Arab jurisprudence clearly extended chattel slavery to Africans! They were fair game in the eyes of Europeans and Arab/Muslims. 

The reason that Africans were taken to the Americas in large numbers, and not elsewhere, is related to the colonial system. Europeans were filling a labor shortage with the easiest population they could find for that reality: African slaves. 

European colonies in Asia, on the other hand, would often rely on local slaves or wage-slaves (indentured laborers), and importation of Africans was not as cost effective.
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Sorry it was a long but I had to explain the whole thing you can pick and choose what u wanna take out