Why does Chopin introduce the reader to her protagonist as "Mrs. Mallard" rather than by her first name?

Because this character denied his identity for a long time.

Explanation:

The Story of an Hour is a tale about freedom in a relationship such as marriage. After the death of her husband, Louis Mallard begins to suffer, not only with his absence but what comes to a woman without a husband in the 19th century. She hides her name because of shame and to avoid criticism or bad looks toward her.