Originally Posted by The Joker
It's nothing to do with it not being done yet. We haven't had Killer Croc, The Ventriloquist and Scarface, Black Mask etc yet either, but I could envision them in a Batman trilogy.
The Joker is Batman's most popular and number one enemy. Aside from the fans wanting him, WB knows he's a cash cow that brings in audiences.
Ehh, I still believe a trilogy can work without Joker. Needless to say, while Joker is a cash cow, TDKR did make more than TDK, so anything can be a cash cow given if the film is good enough.