In order to understand the answer to this, we have to look at what the other choices open to the Europeans were. If they were going to use unfree labor, they had two main choices. First, they could use other Europeans. Second, they could use Native Americans.
At first, they did try to use Europeans. These were the indentured servants. From the point of view of the plantation owners, however, there were problems. First of all, not that many people wanted to come to the colonies as indentured servants. Second, it was easier for an indentured white person to escape and melt into the mass of white colonists. Finally, white servants were typically British and had a strong sense of what their rights were. This made them less pliable than non-whites.
That left either Indians or Africans. In the Caribbean, there were very few natives still living. In North America, there were more, but those natives were not particularly suitable. They, too, could escape relatively easily. Enslaving Indians also meant that there would be a greater likelihood that other Indians would attack to gain revenge or to rescue their fellows. Finally, Indians were not typically versed in large-scale agriculture.
Africans made ideal slaves (in the eyes of plantation owners) because they did not come with any of these problems. They could not run away easily. They had no expectation of having the rights of British people. There was no one to rescue them.
Thus, Africans were chosen mainly for economic reasons in the sense that they were easier to keep in servitude. The decision to use them was not really based on racial issues in the beginning.