The Garleans come from a continent we barely have any access to up until now and have been one of the primary enemies in the game for 12 years, driving the plot by being the bad guys and wrecking stuff.
The three city-states are located where the game takes place, are not our enemies, and are the people we work for so it makes sense for their image to be smoothed out before the Garleans'. Especially since the story started to stop with internal politics and squabbling in Eorzea and concentrated on the bigger threat of Garlemald and the cosmic threat of the Ascians.
It also makes sense that now that the Garleans are no longer our enemy, their own image will be rehabilitated over the course of the upcoming story.
I'm not sure why this is a mystery to you.
