Though recent trade reports have spun the story as being an issue mostly about money, that's pretty much a small part of the motivation. Ross has never been a filmmaker that repeats himself (going from satire in "Pleasantville" to horse racing drama in "Seabiscuit" and action in "The Hunger Games") and we're told the burning desire simply isn't there to spend another couple of years with Katniss in the Capitol (evidently, he also liked the first book best). And while the lowball salary offer probably didn't help, Ross already has a fairly lucrative career as a screenwriter (and rewriter) and money isn't really the issue. Simply put, the filmmaker is looking to change things up for his next effort.
While the details on what that project will be are still coming together, we're told it will be something he has written that (ironically) will earn him a better payday that he would've received on "Catching Fire" (and it's possible it's this "Outback" project) Essentially, it's a win-win for Ross, who gets to move on to a project he developed for his fourth feature, and will allow him to do something different.
The hunt will be on to find a replacement soon. "Catching Fire" shoots this fall and will open on November 22, 2013.