I think it is because too many citizens of African-American origins, especially those who take on the racial leadership mantle, have something to gain by reinforcing this division. It falls perfectly into their game plan. As long as they can continue to segregate THEMSELVES as targets of racial inequality and injustice, then they have a political chip to play in the game, so to speak. As soon as they stop seeing themselves as a separate race with all the inherent baggage that entails, they suddenly lose their "special" status and become just like everyone else. No affirmative action, no outcry of racial descrimination when things don't work out in their favor; in essence, no agenda or basis for special treatment.
There are a lot of African-Americans who have shed this "minority persecution" mindset and have integrated themselves admirably into American society, just like all the racial and ethnic groups who came before them throughout our history. They have become model citizens, rising to the highest ideals of American society. Sadly, there are a great many who will not or cannot follow their lead, who insist on preserving racial division as an easy way to place the blame and responsibility for a multitude of issues they refuse to accept for themselves.