Because the network does not even want Batman himself appearing on the show. The episodes are set before Clark Kent is officially Superman- so Bruce Wayne is more than likely not even Batman yet. Therefore- there is no reason to have the Joker involved in the stories.
The cast of The Joke on the Joker - 1912 includes: Fred Mace as The Joker
The cast of Hollywood Joker - 2008 includes: Matt Cinquanta as The Joker
The cast of Joker - 2006 includes: Bryan Bockbrader as Joker Jeremy Craven as Charlie
The cast of Joker - 1949 includes: Master Bhagwan
The cast of The Joker - 2010 includes: Stephen Cheriton as Stewart
The cast of The Joker Returns - 2010 includes: Matt Cinquanta as The Joker Tatiane de Oliveira as Petta
The cast of Joker in the Forest - 2005 includes: Katsura Kan
The cast of Ha-Joker - 2001 includes: Zvika Hadar as Host
The cast of Jacky - 1990 includes: Gadi Alon as Joker (1976) Anat Barzilai as Joker (1976) Irit Sheleg as Joker (1976)
The cast of Joker - 1999 includes: Masato Fujita Momiji Fuyuno Kazunari Maenishi
The cast of Le joker - 1931 includes: Florelle Marie Bell Marthe Sarbel
The cast of A Joke on the Joker - 1914 includes: Charles De Forrest Vivian Prescott