When White men came to mainland North America, there were an indigenous population already here. I will refer to that population as North American Indians.
Africans were brought to the Americas (eventually) as slaves. Talk to an average White man in the South, point at a group of Black men (African Americans, Negros) and he could be expected to think "slaves".
Why didn't America (or the English before the American Revolution) think of (or write laws to the effect that) the indigenous North American Indian population as slaves?
NOTE: I'm not asking why didn't they enslave American Indians, just why didn't they view them as (think of them as) slaves.