They did originally enslave American Indians, but three things put a stop to it. First, the Indians were all dying of epidemic diseases, which Africans had already been exposed to. Second, the Indians, being native, had a better knowledge of the land and its peoples, which made escape/revolt attempts more likely to be successful. Third, in response to debates about Spain and Portugal's treatment of indigenous people, the pope issued a bull in 1531 that banned the enslavement of American Indians.
