* Protecting Lucy: Mrs. Manette likely believed that the truth about Dr. Manette's imprisonment would be too much for Lucy to bear. It would be a constant reminder of the horrors he had endured and the uncertainty of his future.
* Avoiding social stigma: The French Revolution was still very much alive in the minds of the English. The news of a man imprisoned in the Bastille, and later, his escape and return as a broken man, would have been a shocking story that could have brought shame upon Lucy and her family.
* Preserving the hope of a normal life: By maintaining the fiction, Mrs. Manette was trying to allow Lucy to enjoy a semblance of a normal life, free from the burden of her father's past.
* Protecting her husband's reputation: As a doctor, Mr. Manette was respected in society. Knowing that he was a former prisoner would have brought shame upon him, and potentially affected his career.
* Respecting Dr. Manette's wishes: It's possible Dr. Manette himself requested that the truth be kept from Lucy, perhaps fearing that the news would worsen his condition or hinder his recovery.
Ultimately, Mrs. Manette's actions were driven by a deep love for both her daughter and her husband, and a desire to protect them from the harsh realities of the past. While the truth was eventually revealed, the reasons behind Mrs. Manette's actions are understandable and emphasize the complex dynamics of family relationships in the face of tragedy.