I'd imagine because the "dungeon" is a lot longer than any other, and has the greatest number of bosses.
The level range for Deep Roads starts at a higher level. A gating encounter was put right at the beginning of the Orzammar quest because death rates were quite higher during that plot than elsewhere, and the gating encounter served one of two purposes:
People would get frustrated early and turn around, coming back later.
People would be able to manage the first combat, and as such likely be able to manage the rest of the content (whether because they are patient with save loads, or just particularly strong players).
Easier to do this than to do another full balancing pass over such a large area.