Why did Elizabeth and Jane decide not to expose Wickham after reading Darcy's letter?

In Ch. 40 Elizabeth tells Jane how Darcy proposed to her when she was at Hunsford and how she rejected him. She then tells her of Darcy's letter which contained the information about Wickham trying to elope with Georgiana. Jane of course is shocked and surprised to hear this. Finally, Elizabeth seeks Jane's advice as to whether or not to expose Wickham's mercenary and evil intentions to the general public at Longbourn:

There is one point on which I [Elizabeth] want your advice. I want to be told whether I ought, or ought not, to make our acquaintance in general understand Wickham's character.''

Miss Bennet  paused a little and then replied, ``Surely there can be no occasion for exposing him so dreadfully. What is your own opinion?''

``That it ought not to be attempted.Mr.Darcy has not authorised me to make his communication public. On the contrary, every particular relative to his sister was meant to be kept as much as possible to myself; and if I endeavour to undeceive people as to the rest of his conduct, who will believe me? The general prejudice against Mr.Darcy is so violent, that it would be the death of half the good people in Meryton to attempt to place him in an amiable light. I am not equal to it. Wickham will soon be gone; and therefore it will not signify to anybody here, what he really is. Sometime hence it will be all found out, and then we may laugh at their stupidity in not knowing it before. At present I will say nothing about it.''

``You are quite right. To have his errors made public might ruin him for ever. He is now perhaps sorry for what he has done, and anxious to re-establish a character. We must not make him desperate.''

The reasons are as follows:

1. "Mr.Darcy has not authorised me [Elizabeth] to make his communication public": Elizabeth decides against exposing Wickham because Darcy's in his letter to her has not authorized her to do so.

2."every particular relative to his sister was meant to be kept as much as possible to myself": In fact, Darcy had specifically instructed Elizabeth in his letter that the information about Wickham's intended elopement with his sister must be kept a secret. If Elizabeth had to prove that Wickham was a villain she would have to reveal this information which would certainly spoil the reputation of Darcy and his sister Georgiana.

3. "if I [Elizabeth] endeavour to undeceive people as to the rest of his conduct, who will believe me?"  Elizabeth remarks that she cannot prove Wickham's villainy by any other means except by revealing that he attempted to elope with Darcy's sister.

4. "The general prejudice against Mr.Darcy is so violent, that it would be the death of half the good people in Meryton to attempt to place him in an amiable light. I am not equal to it." Elizabeth remarks that everyone is prejudiced against Darcy and they are convinced that he is not a good person. She despairs that it would be impossible for her to convince them that he is actually an affectionate and good hearted man.

5." Wickham will soon be gone; and therefore it will not signify to anybody here, what he really is." Elizabeth remarks that the Militia will soon leave Meryton and Wickham will also be forgotten by everyone so she feels that there is no point in exposing him as a villain.

6." `You are quite right. To have his errors made public might ruin him for ever. He is now perhaps sorry for what he has done, and anxious to re-establish a character. We must not make him desperate.'' Jane agrees with Elizabeth and concludes that exposing his evil misdeeds to the general public might damage his reputation permanently. She feels that Wickham might have turned over a new leaf now and that nothing must be done to harm his chances of rehabilitating himself as a good person in society.