Answer: Because Mrs. Mallard has a heart condition, and Josephine does not want her to have a heart attack.
Step-by-step explanation:
At the very beginning of "The Story of An Hour", it is stated that Mrs. Mallard has a heart condition, which is why Josephine and Richards have to be careful when telling her that her husband has died:
"Knowing that Mrs. Mallard was afflicted with a heart trouble, great care was taken to break to her as gently as possible the news of her husband's death."
Ironically, Mrs. Mallard is not upset upon hearing the news, but suffers a heart attack and dies when she finds out that her husband is actually alive.
"The Story of an Hour " was written by Kate Chopin in 1894. It first appeared in Vogue, with a title "The Dream of an Hour."