[quote=@Expendable]
[@The Savant] - Perhaps there's something that the Dominion needs, that may have survived out there.

Perhaps they've grown too much? What if they need to make a new city? Only the machine they used to make all the cities was abandoned up there?
[/quote]

The thing currently is that no one, at least no one that is known of, has been to the surface. All the ancient vaulted doors are stuck closed or people do not know how to open them because it's lost technology or they don't have enough time to figure it out because of scavenger groups, wildlife, etc... 

Currently, Dominion is overpopulated like any city is for the actual capacity. Necessities like food, freshwater, and similar things are highly farmed and processed (probably to a negatively impactful point) and the government is very strict. They do not want people wandering outside of the walls, not really, so if you do go outside (you are sneaking or you are such a low rank in society that they don't care to keep track of you). 

Pretty much, societal ranking is: 

Wealthy are idolized for their luxuries. 
Middle class is comfortable and most people strive to be this or better. 
Working class is bearable and most are fine with such a life 
Poverty is... similar to how people see unwanted animals. 

For example, let's say a wealthy person murdered a family in Khia, the majority of wealthy people's reactions would be "It's similar to accidentally running a dog over. It's sad, you wouldn't mean to do it, especially not a second time but you cannot let it ruin your life!" 

Dwellers are the acceptable version of burrowers. Not understood very well but they tend to be "human" enough that no one pays them mind from poverty to middle class. 

Burrowers are criminalized and outcast to the point that if they enter the city, they could be arrested, and put to death. Burrowers are viewed as subhuman and not actual capable of citizenship. 

These explanations kind of show government and people who are comfortable/in control of power wanting to keep Dominion inside the walls of Dominion. Not wanting change because change could mean the loss of power and so on.