It's a horrible question from a horrible train of thought, but why did the colonies import slaves from a continent thousands of miles away when their was a large population of Native Americans nearby?
If I remember correctly, Christopher Columbus attempted to do just that. About the Arawaks he said "They... brought us parrots and balls of cotton and spears and many other things, which they exchanged for the glass beads and hawks' bells. They willingly traded everything they owned.... They were well-built, with good bodies and handsome features.... They do not bear arms, and do not know them, for I showed them a sword, they took it by the edge and cut themselves out of ignorance. They have no iron. Their spears are made of cane.... They would make fine servants.... With fifty men we could subjugate them all and make them do whatever we want."
It seems much cheaper to just take from the tribal populations the colonists were already decimating off and on. Historically, human life was considered the spoils of war from the Romans to the Spanish conquest of Latin America. What am I missing here?
This is a good question, but difficult to pinpoint a simple answer.
You are right, Columbus & co. did use the indigenous populations originally. Eventually though, these indigenous populations began to run thin, and even if they hadn't, they could not nearly provide for the amount of work that the European settlers/explorers required. It was in South America and the Caribbean then that they originally began the African slave trade. It was actually relatively cheap (due to the deplorable methods and conditions used with capture/transportation) to bring blacks over from Africa.
When the Europeans began settling the American colonies, the slave trade was already in full swing in South America and the Caribbean.
Furthermore, a common misconception that many people have in regards to colonial interactions with Native Americans is that the Colonists immediately portrayed their superiority as soon as they got to America. This is not at all the case, in fact, many of the original settlements were failures - specifically during winters. The Native Americans had a well-developed society that functioned just fine in America (after all, it was their homeland). It was the Native American Tribes that needed to help the Colonists survive - not the other way around. Yes, the Colonists' technology was arguably generations ahead; however, they still lacked the capability to survive in this new land. It was the Natives (through the exchange of goods) that helped them through this.
So, the takeaways of this are:
The African slave trade had already been set up in South America and the Caribbean and could relatively easily be shifted north to the colonies.
Native American society was far too advanced to simply just be enslaved by the struggling white settlers.
Because of the gruesome nature of blacks being exported from their society/culture, it was relatively easy to keep them enslaved.
A good summary of this is through this quote from Howard Zinn in A People's History of the United States (and yes I know r/askhistorians sentiment towards this book):
[The blacks] helplessness made enslavement easier. The Indians were on their own land. The whites were in their own European culture. The blacks had been torn from their land and culture, forced into a situation where the heritage of language, dress custom, family relations, was bit by bit obliterated except for the remnants that blacks could hold on to by sheer, extraordinary persistence.