Military strength of tribal nations sufficient enough to be a threat to the young colonies; taking Native slaves could be risky and sometimes costly endeavor.
The African slave market was more convenient and cost-effective, compared to warring with tribes and trying to enslave a people that would either die quickly in bondage, or could escape more readily.
African slaves, once brought to the Americans, were presumed to have slave status by the nature of their racial identity. Thus, it was easier to maintain the perpetual slave status of Blacks. There were few places an escaped Black slave could run and hide out, and nowhere to effectively maintain this freedom for any length of time (Maroon communities in North America were always of a fleeting nature)
Natives were being treated as sovereign nations during the colonial era, and their individual freedom was acknowledge by default.
Racial views varied, and Europeans saw Native Americans in a slightly more positive light - even as they sought to dispossess them and destroy their cultures
