Why is Barack Obama considered the first black president and not the first bi-racial president?

I voted for Obama and fully support him, and am tickled pink that he won the election. With that said the only thing that bothers me is the fact that the press has labeled this an historic election with the first black president. I stand firm on it should be said he is the first bi-racial president. Is it based on physical attributes alone? It must be because he is the epitome of the definition bi-racial don't you think?