Why wasn't Ricci nominated?

I watched this film again for the first time and was left wondering one thing. Why wasn't Christina Ricci nominated for Best Supporting Actress? It's really about a duo for the entirety of the film, that being the duo between Aileen and Selby, and Ricci gives just enough emotional finesse and vulnerability that I really cannot see how she was overlooked. Perhaps everyone was so taken back by Theron's transformation that everyone else just became a background figure. But she did a stellar job here. Roger Ebert nailed it in his review of the film...

Christina Ricci finds the correct note for Selby Wall -- so correct some critics have mistaken it for bad acting, when in fact it is sublime acting in its portrayal of a bad actor. She plays Selby as clueless, dim, in over her head, picking up cues from moment to moment, cobbling her behavior out of notions borrowed from bad movies, old songs, and barroom romances.