That's a good question. Our thinking on race is essentially binary: black or white (or whatever other combination). Up until recently, there was no box on the census for "Mixed-race" (though, again, why is the half white/half black person "mixed?" How is their racial make-up any less distinct than anyone else's?)
I would say, people in our society still have a subconscious belief that when a white person has a child with a black person, the resulting offspring is somehow a diluted white person (why don't we consider them to be a diluted black person?), less "pure" than a white child.
Why isn't she considered a white actress?
Because, on the one hand, most people in our society probably believe that having any black blood automatically makes a person black. On the other hand, race-hustlers and members of the diversity industry have an interest in emphasizing race in America.
I think a more important question is: Why isn't she simply labelled an "actress?"