Because he CLAIMS to be black. As to what he is who knows, if we accept his claim as to his parentage he is at minimum 1/2 Caucasian and possibly as little as 1/16 black African with the remainder Arab.
I am old enough to have gone through 12 years of segregated schooling and I remember at least one kid who attended the white school who had darker skin than Barack Obama.
I’m pretty sure Obama has little or no Arab ancestry. Barack Obama, Sr., was an African tribal and a Christian who converted to Islam, assuming BO,Sr., was indeed his father.