And here's the thing: The Great Divide comes in Season One, when we're just getting to know the world of Avatar. It kind of makes sense for us to have a mostly self-contained filler episode that gives us a look at two ethnic groups and their unique dilemma, since we're being introduced to how this world functions.
By Season Three, we KNOW how the world works. Sure, you can argue that we need to see what the Fire Nation is like, but more than anything, Season Three needs to be about plot. We need to see Aang learning firebending, we need to see Zuko clashing with Ozai and turning against him, we need to see the wrapup of the character arcs of Katara, Sokka, Toph, and Azula. Season Three was NOT THE TIME for an episode like The Painted Lady. It was horribly out of place, along with a lot of the filler-y aspects of Season Three. Bryke were dicking around in their world when they needed to get on with telling the main story.