Spoilers from Book #3
I'm including a spoiler warning since this goes over all three books. Turn back if you don't want to know.
Well, Silo One has elevators and no stairs. The rest has stairs and no elevators. The reason I took from the books is that the no-elevator-reason stems from two things. Slow down communication, uprisings and all of those in the silos for one. Thus, reason one is simply controlling the people.
However, the author replied on his blog to such ideas as follows.
The real reason is more nefarious than that. - Hugh Howey
The reason is that the Silos Two to Silo Fifty can be terminated remotely by releasing some airborne killer, the white smoke. That smoke is designed to propagate via the stairs. Reason two is killing the people.
And then she saw the white clouds like smoke higher up. Maybe from the Mids.
More screams This time from those nearby, those spilling down the stairwell, the few who had made it. [...] And the white smoke, it descended down the stairwell like a hammer.
Silo One on the other hand has no stairs so that nobody notices how big the concrete levels in between are. For the new world order no one can know what happened. The same termination method would make Silo One people very uncomfortable, thus it's rigged to blow. Explanation from the engineer (Donald) in Dust.
"We're rigged to blow," Darcy said. "There's the same mass of concrete between every level." Donald nodded. This kid was fast. "If they let us walk up a flight of stairs, we'd see. We'd know. [...]"