In her letter in ch.52, Mrs. Gardiner says that Darcy blames himself for Lydia's elopement, because if he (Darcy) had let the world know about Wickham's behaviour, no one would have trusted him. Still, what could Darcy have said about Wickham without having Georgiana's name dragged into it eventually? Knowing Wickham, I would be afraid that in retaliation he would start dropping hints himself, and trash Georgiana's reputation by innuendo.