Why does Madame Forestier fail to recognize Mathilde?

After Mathilde Loisel borrows Madame Forestier's presumably expensive necklace to wear during an aristocratic ball, she ends up losing the necklace, which she believes is authentic and made with genuine diamonds. Mathilde Loisel and her husband end up buying an authentic replica of the necklace, which costs them thirty-six thousand francs. In order to afford the extremely expensive necklace, the couple spends their inheritance and borrows the remainder of the money. Mathilde Loisel and her husband move out of their apartment, get rid of their servant, and Mathilde ends up working strenuous jobs in order to pay off their debt. It takes Mathilde Loisel and her husband ten years to pay off Madame Forestier's authentic necklace. Guy de Maupassant writes,

Madame Loisel looked old now. She had become like all the other strong, hard, coarse women of poor households. Her hair was badly done, her skirts were awry, her hands were red. She spoke in a shrill voice, and the water slopped all over the floor when she scrubbed it.

When Mathilde approaches Madame Forestier walking along the Champs-Elysees, Madame Forestier does not recognize Mathilde because she looks so much older and unhealthy from the years of arduous labor. Ironically, Madame Forestier informs Mathilde that her necklace was an imitation worth only five hundred francs.

Madame Forestier doesn't recognize Mathilde because she's changed so much since she last saw her; and there's a very good reason for that. After the unfortunate incident with the lost necklace, Mathilde's husband was forced to take out a series of crippling loans to buy a replacement. Inevitably, this meant that the Loisels' standard of living plummeted and they were forced to move out of their apartment into more modest quarters.
In order to make ends meet, Mathilde has had go out to work, and leads a life of mindless drudgery far removed from the opulent, aristocratic lifestyle to which she always believed herself entitled. When Madame Forestier catches up with her on the Champs-Élysées, Mathilde presents a pretty sad sight. Due to years of grinding poverty and hard toil, she's become a shadow of her former self, prematurely aged, and wearing shabby clothes. She couldn't be more different from the glamorous, beautiful woman who was once the undisputed belle of the Education Ministry ball. No wonder Madame Forestier doesn't recognize her.