It may seem ludicrous to us, knowing as much as we do, but Darcy does not have the advantage of the reader. At this point, we don't know what he was thinking when he tried to persuade Bingley to change his mind--or even how much Darcy was involved in the persuasion at all. Remember, Darcy wanted to define the perameters of the argument about persuasion more closely, but that never happened.