The difference is that the Fereldan civil war was never the central focus of the story, nor was it ever meant to be. It was a device that kept the Warden from going to Queen Anora and getting Fereldan's army lined up to fight the blight. Honestly if you listen to the gossip, in DAO you learn that for the most part the Fereldan Civil War has all the power of a wet firecracker (with the exception of a battle and incident or two). The reason is clear. After Ostagar, there aren't that many soldiers and the nobility is holding back for fear of the Darkspawn....so it's hard to fight a civil war without soldiers (which explains why Loghain resorted to slave trading in the end).
By contrast, the Orlesion Civil War was supposed to be one of the central themes of the game (along with the Mage-Templar war)...but we don't see it or really hear any rumor about it except perhaps on two maps that are optional anyways.
First, the Orlesian civil war is not a "theme." DAI's "themes" are an exploration of faith and the corruption of institutions meant to protect us.
Second, the Fereldan civil war is almost entirely irrelevant to the plot. It does create the odd side quest, but mostly it just ties Loghain up so that he can't make an earnest effort to hunt down the Wardens. The slavery subplot could still have occured as Loghain struggled to restore what was lost at Ostagar.
Most of what occurs in three of the five optional maps in Orlais (not counting the Forbidden Oasis or the Hissing Wastes) occurs as a result of Corypheus' men taking advantage of chaos caused by the civil war.
1) The Exalted Plains, obviously, are a grisly battlefield, filled with burned-out shells of buildings and corpse-strewn trenches. But the war has allowed the Venatori to create a small army of the undead, and deserters from both armies have banded together to form the Freemen of the Dales.
2) The Emerald Graves sees refugees from the war band together under leadership of Fairbanks to fight the Freemen, who are trying to claim the area to declare their own sovereign nation. The Red Templars are using the Freemen in turn to move red lyrium through the Dales.
3) Sahrnia in Emprise Du Lion was facing economic ruin as a result of the war when the Red Templars took advantage of their desperation to purchase the local quarry, which they used to grow red lyrium.
In all these areas, you hear from NPCs or find notes of people who have lost loved ones to the war. Finally, it all comes to a head in Wicked Eyes Wicked Hearts, where the Venatori try to use the event to eliminate all of Orlais' leaders.
No, you don't actually see any fighting between Gaspard and Celene's armies, and that's because a ceasefire has been called so that peace talks can go ahead. But you never saw any actual fighting between Loghain and his opponents in DAO, aside from ONE side quest.