Let's back up. Bright Star is Jane Campion's account about the love affair between John Keats and Fanny Brawne. She was inspired to write the film after reading a biography of the poet (written by Andrew Motion) as well as Keats' own poetry. However, the film is told from Brawne's point of view, and there isn't a whole lot written about her. In short: Campion imagined what their love affair might have been like after reading a number of sources. In even shorter words: She researched and filled in the blanks creatively. Nevertheless, her work has been deemed an adaptation, as if she just read someone else's work, and adapted as she saw fit.
Unless Campion is lying, and there's this big, secret Keats/Brawne work that we all don't know about, how on earth could they come to this decision? Last year, Dustin Lance Black won for Milk, and without a doubt, he researched his subject, and didn't create it all out of thin air. And he also, most definitely, had a lot more source material to work with. In 2007, both Letters from Iwo Jima and The Queen grabbed Original Screenplay nominations. Syriana was inspired by a true story, as was Hotel Rwanda, The Aviator ... you get my point.