Because they were repetitive, not because they were long.
For a repetitive combat-centric dungeon, DAO's Circle Tower is about as long as you should go. I'm talking about just the tower, not counting the Fade.
But longer dungeons can work well if there's a greater variety of activities. I've said before that I think DAO's Deep Roads were too short, but I wouldn't want to add any combat to them. Spread out that same combat (or less) over a dungeon three times that long. That would create the sense of scale and loneliness the OP wants, while avoiding the drudgery some people fear from long dungeons.
Well, I do sort of like the general notion of what you suggest, but I'm not convinced about the conclusions.
First of all, the people who disliked the long dungeons in DA:O generally belonged to a group of people who generally represented a collection of opinons which were rather contrary to, for instance, my set of opinions. So I suspect it might be a much greater factor, and difference, in personality, behind these preferrences. Meaning, - it's not that easily resolved.
Then, a lot that attracts me (and the OP, it seems) is the dread. The dread of endless difficulties, claustrofobia, "will I ever get out?".
And the amount of "drudgery" is important in building that mood. Now I'm not sure "drudgery" is the right word. I assume it's a word chosen only for its derogatory qualities. I say so, because the critics of the long dungeons otherwise come across as people who mainly play video games for the sake of repetetive combat. They seem to have no problems with it above ground. No, I'd say that the 'quality' here, is in feeling 'uncomfortable', and in having an 'uncomfortable' set of tasks, a mountain, ahead of you, - whether those be described as boring, tedious, repetetive or difficult, demanding - before seeing any kind of conclusion of the 'uncomfortable' situation.
I revel in having that kind of experiences inside a cRPG. I loved the Fade and Deep Roads in DA:O for giving it. And I have no problems with it in replays either. It's still as "fun".
So I'd guess it's more if we enjoy the experience of being 'uncomfortable' in the game or not. And that is probably a just as powerful divider as some other japanese/western, DA2/DA:O -differences.