]Lizzy mentions Darcy's proposal to Jane, and also about the letter. But she supresses all information about Bingley.

]She obviously feels that Jane would be more upset if she came to know that Bingley had really cared for her. But I also wonder if she wanted Jane not to think ill of Darcy at that point? What do you all think?

I think this is a possibility. Even though I don't think she has any open regard for Darcy at this point, she had to admit to herself that the content of his letter was justified (she never knew herself, etc.), and that alone would make her want to act more kindly towards him. After all, she has never said anything positive about Darcy to Jane before, and now that she's changed her opinion about his character somewhat, I think she leaves out the Bingley story partly because of Darcy's involvement.