Why was Jim lovell not allowed to command the mission?

Jim Lovell was not allowed to command the Apollo 13 mission due to a serious medical issue. He had been exposed to a viral infection that caused him to experience significant health complications, leading NASA to replace him with Jack Swigert as the command module pilot. This decision was made to ensure the safety and success of the mission, as Lovell's health was a concern. Ultimately, Lovell still played a crucial role as the commander during the mission despite these challenges.