Why is my lawyer waiting to send a demand letter?

A persuasive demand should present a coherent account of fault, injury, treatment, lost income, future needs, and supporting evidence. Counsel may wait until treatment is complete or the medical prognosis is sufficiently clear—sometimes called maximum medical improvement—so the demand does not omit surgery, rehabilitation, permanent restrictions, or future earning loss.
Waiting should have a reason, not be unexplained inactivity. Ask which records or medical opinions are outstanding, whether an earlier demand would be useful, and how the filing deadline is being protected. In some cases a lawsuit must be filed before a final demand package is ready.