At the Oscars, Daniel Day-Lewis won Best Actor.

How so?
"Lincoln" doesn't have any main black characters. It has hardly any black people at all. Sure, it's about that President at a specific moment, and not about slaves. But what and who is Lincoln? Why do we honor him today? States' rights were at stake, but the civil war was fought over slavery, and that is Lincoln's legacy.
All during the film, black people are peripheral, somewhere on the edges, rarely seen, rarely on screen. And aside from gentle scenes of dialog at the beginning and end of the film, they are never confronted. Their captivity and freedom is debated with a little input from those most affected.
But though they are invisible, black Americans are everywhere in the themes and culture and gravity of the moments being enacted. They are the thing referred to but never spoken of; they are exchangeed but never valued. They are marginalized in the film, reflecting their existence in America, and the racially collective experience of their existence here for some time.
Notes:
* The only other film I saw is "Flight".
(Also, the President now is black.)