Quentin Tarantino explains why Frank Ocean isn't on the 'Django Unchained' soundtrack
Tarantino released a statement about the omission:
Frank Ocean wrote a fantastic ballad that was truly lovely and poetic in every way, there just wasn't a scene for it. I could have thrown it in quickly just to have it, but that's not why he wrote it and not his intention. So I didn't want to cheapen his effort. But, the song is fantastic, and when Frank decides to unleash it on the public, they'll realize it then.
UPDATE: Frank has released the track on his Tumblr.