I'd imagine because the "dungeon" is a lot longer than any other, and has the greatest number of bosses.
I'd argue the Circle Tower is just as long, if not possibly longer, than the Deep Roads and contains two Big Bosses, numerous mini bosses and a high volume of combat. Yet it is the most often suggested area to visit first.
The Brecilian Forest contains over six maps, the same amount as The Deep Roads. And has numerous encounters, including a fight with a dragon (the only non-High dragon fight you can encounter, when the game only lets you fight a High Dragon in Haven or confronting Flemeth, both opportunities that are follow ups visitng one of the Primary Quest hubs, all but guaranteeing you are at least at mid-game) as well as a stiff end-game fight with either The Lady of the Forest or Zathrian.
And Redcliffe is often criticized because of its high skill checks and difficult combat as being a terrible place for a character like Alistair to suggest the player visit first, since it is often a challenge they are not able to address with a high rate of success.
And that's one thing that I like about DA:O - sufficient challenge at every step of the way. I'd like it even more if the entire game was given the Redcliffe treatment, when the player encountered many things in every instance they might not be strong or skilled enough to overcome. Which it seems DA:I is doing, so that's very cool.