The reason that black people support the Democratic Party in such high percentages is because black people are disproportionately lower-income and disadvantaged, and the Democrats have set themselves up as the party of the working class against the rich. (Not altogether true, except compared to the GOP.) Even those black people who are not lower-income either have been in the past, or emerge from African-American communities including a lot of people who are. Also, the history (past and present) of slavery, segregation, and official and unofficial discrimination gives black people in this country a natural inclination to take the part of the economic underdog.

Given all that, for a black person to be a Republican almost always requires that he be, not only financially successful, but also willing and inclined to sever himself emotionally from the African-American community and from the history of black people in this country. Since only a relatively small minority of African-Americans fit that description, the lopsided support is not very surprising, or shouldn't be.