What the other characters didn't understand about The Joker.
The Joker wasn't about money, as we see in one scene he literally burns a tower of tons and tons of cash, Batman lost it in one scene when he was beating The Joker up in the police station and yet The Joker just laughed at him, and as The Joker said "You've got nothing to threaten me with, even with all of your strength". It took them near the end of the movie to realise what The Joker was all about, he just liked to cause chaos between people, that was what he was about.
It was clever that they didn't have a psychologist in this flick to sum The Joker up, if they did they would have known his motive and shut all that crap that he was trying to pull down, and the film would have ended a lot sooner.