Necessity is the mother of invention as they say. I think the reuse of assets was the meme that came out of DA:II, but I still think each area of the city areas had their own thing going, from the port area to the honey-comb mines turned slums; and even having the nicer parts of the city, still have the kind of oppressive feel of a slave-quarry/turned city-state, worked for me.
I haven't played DA:II in some time though, so I'm certainly adding a little to it on my end. Regardless, the concept of a building a world 'tall' instead of 'wide', and then adding to it a story that takes place over a longer period of time, is still interesting to me, especially because you don't see it very often in RPGs for some reason.
Most likely, the point is not to get the player bored with too much monotony of areas.
I suspect this is the main reason because reusing areas would make it easier for developers.