This is an interesting observation. Faulkner could very well have chosen to use "Miss Emily" in his title, but he did not. Since authors are very focused and selective in writing titles for their works, his choice wasn't careless or without purpose. Faulkner's diction in the title points the reader toward themes in the story.
Emily Grierson is a Southern woman born into a once wealthy, influential, and socially prominent family; her family's fortunes and once grand lifestyle are gone, but the Grierson name separates Emily socially from the "common" citizens of Jefferson and demands that she live her life according to strict standards of circumspect behavior. "Miss Emily" is expected to live up to her name, which makes her a prisoner of the past and Southern traditions.
As the story unfolds, however, Faulkner reveals that the "Miss Emily" the town knows, watches, gossips about, and judges is, in fact, a complex woman who has suffered from the weight of the Grierson name throughout her life. "Miss Emily" is a Southern lady from a fine family; Emily is an isolated, lonely woman, desperate for love and driven to madness. Who she is, really, is a secret from the town. Faulkner's title points the reader to the private woman behind the public face and to the tragedy of her life. It is Emily, not "Miss Emily," who deserves a rose.
The author of "A Rose for Emily," William Faulkner himself explained his title,
The title] was an allegorical title; the meaning was, here was a woman who had had a tragedy, an irrevocable tragedy and nothing could be done about it, and I pitied her and this was a salute…to a woman you would hand a rose
Calling Emily Grierson by her first name rather than "Miss Emily" grants her the commonality with all of humanity that she deserves. It also frees her from the patriarchal Old South landed gentry society to which she has belonged. For, as Miss Emily, she was imprisoned emotionally as she was subjected to the will of her father along with her station as "a tradition, a duty, and a care" of the townspeople who referred to her in this way.
With the use of the word rose occurring twice as a verb, first with the aldermen having risen after Emily has entered the room when they have come to collect taxes, and then in the last section when the dust of the room "rose" about the people's legs, the connotations of this verb both suggest the tragedy of Emily's reclusive life, a life lived in the past. While all is green outside, Emily has lived within the confines of her house, seen only rarely. In a sense, Emily has been married to death; so, she poisons Homer to keep him, too, inside with death in the rose-colored room, suggestive of the sunset. The "rose for emily" is like the rose one places on the coffin as it descends the grave, a final sentiment to one who has experienced the tragedy of never having truly lived freely. Calling her "Emily" rather than "Miss Emily" finally frees her from her social imprisonment.