... that Lizzy told Wickham when she talked to him at the party that Darcy had been in the area for a month. I think that, before Wickham started pouring poison into her ear, Lizzy was not unalterably opposed to Darcy. Had he been consistantly civil to her, he might have been able to turn her opinion around. (I wouldn't say that he was high on her list of men she would have wanted to marry, but a woman in her position did not cross such a eligible man off of that list lightly.) However, once Wickham told her his story, and she believed it, she came to believe he was dishonorable. And she would not marry a man she believed was dishonorable.
At least, that's my theory.