I was wondering why (almost) only sub-Saharan Africans were brought to the Americas during the slave trade, given the extent of European colonies at the time (which would mean people from other ethnicities could be enslaved, I assume). I'm aware that in some countries there was also Amerindian slavery, but my doubt regards only slave trade. Here in Brazil we're taught that slave owners preferred black slaves due to the idea that they were "better workers" than amerindians. Is the same idea involved in the "preference" for black slave trade or is it something else?
This has in part to do with the trade networks that existed between Europeans and various groups in Africa throughout time. The big European "players" in trade with African peoples were the Portuguese, British, French, Spanish, and Dutch. When Europeans began trading with African peoples, they were exchanging goods, but eventually the European demand for slaves in general grew as each of the major powers was competing to expand and claim areas of the new world. This led to slaves being one of the most traded commodities in Africa. Many African peoples, including those in Sub-Saharan Africa also engaged in slave owning within their own regions. Slaves were often captured in battles with other peoples, or were enslaved because they owed a debt to someone else. Captive slaves were often considered outsiders (though not in the same ways that slaves were considered "others" in North America, for example), so the sale of slaves to the Europeans wasn't necessarily an immoral thing like it might be seen from a different/modern perspective.
In terms of why Sub-Saharan Africa specifically, that could have to do with the resources in the region, and the desire to establish trade in West Africa/the desire for direct acquisition of resources there rather than buying goods which were transported through the trans-Saharan trade routes. Portugal for example established a few trading ports in West Africa-at Cape Blanco, Elmina, and Sierra Leone for example (and also sponsored Bartholomeu Dias's expedition to round the Cape of Good Hope (in order to establish direct trade relations with the Far East). Sub-Saharan Africa just happened to be the region that was the most advantageous for them.
I think it's important to note that the trans-Atlantic slave trade happened for hundreds of years, so the political/social/racial dynamics which influenced trade change over time (additionally, they are quite different depending on which Europeans and which African peoples you're looking at).
This book might be of interest to you. If you go to university, you may be able to access it with your institutional log-in/through your uni's library (I could): Africa and Africans in the Making of the Atlantic World, 1400–1800, John Thornton, Cambridge University Press, 1998.
(Also this is my first comment on here. I'm a history student but by no means an expert. Please don't burn me at the stake if I messed up!).