User blog comment:Godisme/Death & Strawberry 2/@comment-28861328-20110814220924

Rukia's promotion isn't so far out there if you think about it. She was a mere unseated officer, who not only knew her Shikai, but fought and defeated an Espada (the Ninth), another Arrancar near the Espada level (Rudbornn Chelute), and at least put up a fight against the Number 0 Espada, even if she didn't last too long. She is on good terms with her captain and the lieutenant's spot was empty. I think it is perfectly logical why Rukia became the lieutenant.