Why was Miss Emily viewed as a "fallen monument"?

Miss Emily represents the American South as it was before the Civil War.  She comes from wealth that was built on the backs of slaves, and that wealth is now gone.  She is the last of the Griersons. Her house and her person are both symbolic of how the South has "fallen."

The house in which Miss Emily resides is a mansion that has clearly seen better days. It has "cupolas and spires and scrolled balconies...on what had once been our most select street" (1). It is now in "coquettish decay" (1), an "eyesore among eyesores" (1).  When the townspeople go to try to collect the taxes that Miss Emily owes, they are admitted to a house that "smelled of dust and disuse - a close, dank smell" (1). 

Miss Emily herself deteriorates throughout the story. She is obese and "bloated" (1) during this first visit to the house. When her father dies, she shows clear evidence of mental deterioration, denying that her father has died.  Her hair begins to turn gray, and she remains obese.  For the most part, she remains sequestered in her house, with the notable exception of her purchase of rat poison.  She never is financially able to pay her taxes, she has shut off large parts of the house because she cannot maintain them, and she dies alone in a downstairs room, "her gray head propped up on a pillow yellow and moldy with age and lack of sunlight" (5). 

The specter of the Civil War hangs heavy over this story. After the war, Miss Emily is "a fallen monument" (1), living in a house that is fallen, too.  The demise of the ruling class of the South is represented in Miss Emily, who is the last in her family, and possibly the last of that generation in her town.