Why don't the loisels tell mme forester that they lost the necklace?

The Loisels do not tell Madame Forestier that they lost the necklace because they were ashamed and afraid of the consequences. They were worried about how Madame Forestier would react to the news, given the value of the necklace and their inability to replace it. Additionally, they hoped they could somehow find a way to replace it without her ever finding out.