Why didn't Emily accept that her father was dead for three days?

Miss Emily's refusal to accept the fact of her father's death is a miniature version of her failure to accept the changing world of the South. Her family's wealth and prestige was lost in the Civil War, as was the culture of the antebellum South. As the years go on and things change more and more, Emily stays, aggressively, the same. 

An additional interpretation to Emily's reaction could be that she was in a state...

Miss Emily's refusal to accept the fact of her father's death is a miniature version of her failure to accept the changing world of the South. Her family's wealth and prestige was lost in the Civil War, as was the culture of the antebellum South. As the years go on and things change more and more, Emily stays, aggressively, the same. 

An additional interpretation to Emily's reaction could be that she was in a state of shock after her father died. Certainly many people would struggle with the passing of their parents but Emily in particular has good reason to have problems after her father died. He was a controlling man, who worked to keep her isolated from the rest of the town. After all, when the narrator discusses the town's reaction to her father's death and her response to it, they say,

"We did not say she was crazy then. We believed she had to do that. We remembered all the young men her father had driven away, and we knew that with nothing left, she would have to cling to that which had robbed her, as people will."

Her father, who the townspeople picture in a tableau standing behind Miss Emily with a horsewhip in hand, has had so much control over her life, and kept her from so much happiness by denying her a husband and family and place in the life of the town. The narrator is describing a sort of Stockholm Syndrome, where Emily clings to an abusive father because she doesn't know what else to do.