How come Obama isn't referred to as a biracial president?

I completely understand the relevance of Obama being of Kenyan decent, but why the emphasis on being black? He is just as much a white man.|||Look up "one drop rule." In the US, historically, having any known black blood was sufficient to be considered black.|||Think he has titled black completely because of his wife and children, and that is great!|||I'm sure he will be once a candidate stands half a chance of being elected who is all the way black.