Why are all black people slaves why not white people?

Because during the triangular slave trade most slaves were imported for the north western coast of Africa, also known as "the gold coast" due to the huge amount of profit that slave trade made.

There were white and Asian slaves in the US not just African during that time period. African tribes captured slaves from warring tribes and sold them to America. The ones not sold were killed off. All races of the world were slaves at one point in time; this can be shown throughout the Roman Empire. Also Indians(not "American" Indians) were the highest populous that were slaves in the whole world

All ethnicities have been enslaved, not just black Africans.