The enslavement of the indigenous peoples of the Americas was common and widespread from the moment of Columbus's first voyage. When indigenous labor began to be inadequate, largely because supplies were running low, Europeans began to import Africans to fill that role.

In the colonies which would become the United States, white populations generally were not militarily powerful enough to enslave local people on a large scale. Also, especially in Virginia and Maryland, the use of convict and indentured labor was common early on. Then, Africans were introduced, mirroring the wider trend towards the use of Africans.

By the time of the rise of cotton in the early 19th century, African slavery was the preferred labor system in the American South, so that's what cotton plantations used.

ETA - I think the post linked to by /u/OldWorldGlory (currently below mine) is a more complete response, so I point you there in addition (or in preference) to my own comments.

You might be interested in a past post on this topic:

• Why weren't there any Native-American slaves?
  ^(12 Dec 2015 | 16 comments)
  ^(/u/anthropology_nerd gives a detailed overview of Native-American slavery in the Americas, with comparisons to African-American slavery in terms of its slave trade, effects on populations, etc.)

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