And as well-meaning and generally great Dorian is, he himself admits that he didn't really give it much thought to slavery in his country and in fact the one or two times we can talk with him about it, he shields himself from the issue by pointing out treatment of impoverished and disfranchised in the South.
He also admits to Sera that he'd never even spoken to elves who weren't slaves before he came down south...but is happy he has now. All things considered, Dorian handles the culture shock pretty well.... some very ill-advised words to Solas notwithstanding. If he can't be made to personally answer for slavery in Tevinter, at least he doesn't feel compelled to continue defending it, not like Bull constantly making excuses for the Qun.
When Solas demands he prove he really is sorry by freeing the slaves of all races imprisoned in Tevinter, Dorian admits he doesn't think he can. That's telling: when put on the spot, Dorian doesn't try and argue that being poor in an alienage is equivalent to being enslaved. He doesn't argue the principle. He just doesn't think such a thing is possible. It's definitely beyond HIS power. Solas then says what good is his regret then.
It will be interesting to see, in DA4, how Solas might react if the player CAN potentially abolish slavery in Tevinter. On one hand, it helps Solas because now his elven agents can move a lot more freely. On the other, it's potentially yet another test of his resolve.
Someone should bring up to him that the story of a mage who once destroyed the world waking up thousands of years after the fact, looking around at a present he doesn't recognize and resolving to destroy the world all over again to try and make things just how they used to be....isn't that exactly what Corytheus did? How did that end for him?