A major factor in most 4X games is citizen happiness. If you tax them too much or the cities become too crowded, then the populace might rebel.
Oddly, this feature is absent in Elemental so far.
I guess you could handwave it away by claiming that in the postapocalyptic people are happy if they can stay anywhere.
Still, my citizens are only present as number that determines the level of my cities and as pool for recruiting soldiers, content or at least indifferent to my rule.
Keeping your people happy adds another decision making item to the agenda - balancing your people's needs with your quest for conquest, diverting valuable resources to keeping your people reasonably happy.
To put it a step further - I also think that if people are unhappy with your rule they might not only rebel but also migrate away from your lands, depopulating your cities ... and that if your population drops sufficiently, the city level drops. If a city drops in level, buildings that should require the higher level should stop providing their benefits. This would add more flavor and richness to the narrative:
"Greenfilelds, once a proud city of splendour, is now, thanks to Lord Syt's tyrannical rule, a shadow of its former self, its streets deserted, the buildings in disrepair."