When Columbus and his entourage landed in the Americas they were looking for an alternate route to the East Indies. What they found was an area that was teeming with Native Americans. And although the elaborate truth is not expounded widely, Lies My Teacher Told Me, by James Loewen, gives a detailed account of Ferdinand Columbus's biography of his father as well Bartholome' De La Casas diary. In these accounts vivid and horrific details regarding the enslavement of Native Americans paint a dismal picture of the reality of the interactions between Columbus and them. The reality is that the Native Americans, in the Americas as a broad expression, WERE in fact enslaved by Columbus and his men. There were as many as 8 million Native Americans when Columbus landed in the Americas. But between the cruel way they were treated, diseases that were introduced by the Europeans and the Native Americans refusal to remain subjugated (Many committed suicide and killed their own young to escape the horrors of slavery) slavery could not succeed. The bottom line is that the Native American population was eradicated due to the formerly mentioned reasons. Native American slavery did exist prior to the slavery of Africans. It was simply unsuccessful.
It should also be noted that other Europeans were held as slaves for a period of time as an evolution of the indentured servant system in the area of the Americas we now call the United States. However, due to the ability of European indentured servants or (eventually) slaves to use their complexion and appearance to elude owners and sponsors, and the lack of willingness and resilience of Native Americans, Africans with their stark contrasting complexion and ability to endure adverse conditions well were easy to identify when they ran off and made the idea of slavery easier.
Lowen, J. (2007). Lies My Teacher Told Me. The New Press. New York.
When the "New World" was discovered the African slave trade was already well underway in European countries, as well as in Africa itself. American Indians were never exactly chosen for slavery, but the idea was not completely missing. In Christopher Columbus' journal he remarked upon his arrival to the Caribbean that the men and women there were savages and could be easily dominated.
However the relationship in the regions were often used as more of a friendly neighbor type system. The Europeans (except the Spanish) did not come as conquerors but more as settlers and thus were not primed to dominate a foreign people in a foreign land they did not already have control over. This meant that it was more beneficial to trade with and work with the Native Americans rather than attempt to subjugate them.
Once the settlers grew in power and force, they used their ability to move their 'allies' out of previously owned lands and into territories that were not in use. This led to events such as the Trail of Tears under President Andrew Jackson. African slaves were never used primarily for trade because the thought of working alongside them was foreign, and the peoples of Africa had been subjugated long before the first Europeans began colonizing North America.
In South America the Spanish were planting their roots in the late 1400's. Slavery was already on the way out by this point and Spain completely eradicated slavery in 1542. Slavery was not their style, instead they completely dominated the Incan people they found on their conquests.
Actually, it depends on what ethnicity of colonists you are talking about. If you are talking about Spanish "colonists", they did actually enslave the Indians. However, it soon ended because they realized that the African people were much better workers than the Indians. Plus, the Colombus Exchnage brough many diseases to the Indians, which made them even weaker and unable to work. The Africans had great immunity to diseases, so they were the ones who eventually became slaves.
When Columbus came to the Americas, he brought plague and torture with him. When Europeans began to settle there simply were not enough Natives left to be successful in their work. Africans were the best answer for them because they were mostly more resistant to European illness. Europeans also had more control over African slaves because they were taking them out of their element and placing them somewhere where they had to depend on their masters for survival.This allowed Europeans to have more psychological control over their slaves because they knew that they had nowhere to go and they had to stay with their masters and work.