Batman v Superman is already overstuffed with characters. Currently, it’s got everyone from Man of Steel, Batman, Alfred, Wonder Woman, Lex Luthor, Aquaman, the Flash... and originally there were going to be even more.
Speaking with Collider, Snyder said:
The Joker and The Riddler both came close to being in this movie. We talked about the possibility of putting them in. [Screenwriter] Chris Terrio and I talked about it a lot and felt that their mythological presence is felt in the movie and that’s cool, but I didn’t want to get my eye too far off the ball because I needed to spend the time, frankly, with Batman and Superman to understand the conflict.
The trailer has hinted at Batman’s past altercations with the Joker, but whatever “mythological presence” (sidenote: ugh) the Riddler has is still to be seen. I’m betting their appearances would have been in flashbacks to Bruce’s history as Batman. Which would have definitely enriched it, but, yeah, would have probably cluttered things up unnecessarily.
Probably a good thing the Riddler and the Joker sat this one out.