User blog comment:ImperiexSeed/Supernatural Mysteries/@comment-3481504-20121026193548/@comment-1509826-20121026202020

Because if Lucifer had directly killed bobby, it would have presumably driven sam even further away from saying yes, I mean bobby was like a father to him and to kill him would permently destroy even the little sympathy he had manged to build up for him, plus it could have driven dean to say yes to michael in fury, with the angels promising to bring him back when they win.