Here's my answer to an earlier thread explaining why African slaves were used instead of enslaved natives in the American colonies.

In brief, African slaves were primarily taken due to a utilitarian consideration. North American native populations simply weren't abundant enough to kickstart widespread slave and plantation economy. In addition, two other factors played into the economic utility of using African slaves.

First, Europe, during the age of exploration, utilized sailing as its primary means of exploration. The most readily available path for them, which didn't involve going around the cape of Africa, was to sail directly west, across the Atlantic. Consider that the Atlantic is bordered by 4 continents: Africa, Europe, and the two Americas. This meant that Africa and Europe were more readily accessible to colonizing powers as points of import to the colonies than, let's say, Asia. This meant that for import to the Americas, Africa stood as the best and most easily available source of slaves. The need for slaves over simply enslaving the natives is outlined in my linked answer.

Second, Africa had a booming slave trade. European merchants trading on the African coast easily came across humans as commodities, sold in many cases by communities who had captured said slaves in tribal wars. This meant that there was an extant market, ready to fulfill the demands of the fledgling plantation economies that would eventually form in Brazil, Argentina, some of the Caribbean (the most prominent example of this would be Haiti), and much of North America.

As for the racism. I don't want to go into detail, since I'm not an expert, I'd leave it for someone else. But having written about the logistics of the transatlantic slave trade leaves me sort of obligated to give a primitive and short assessment as to the relevance of racism. In short, the general European attitude to natives of any foreign region was that they were different, most importantly they were pagans. Hence, much of the early justification of colonization and the subjugation of the natives was a sort of 'crusade', which was followed by widespread conversion efforts. Though all natives were perceived as barbaric, it is my understanding that the idea of black racial inferiority developed very specifically as an idea which justified both their enslavement, and then the later colonization of Africa. But it did come after said slave economies began to develop.

u/Georgy_K_Zhukov has written extensively about social attitudes towards blacks in the southern states, I recommend you check out his numerous answers in relation to American slavery if you want much more in-depth analyses of the development of American racism.