Bad writing. On top of that, the book doesn't make much sense to me.
Along with the writing, more reasons to have disliked the book:
1 - Why the staircase?
The silo in which the story takes place has one mode of transportation along its whole one-mile height: a small spiral staircase. There are no elevators - even freight elevators are absent. Why? Doesn't make any kind of sense. Suspension of disbelief ruined.
2 - Dystopia not believable
The author chose to motivate the reason behind it as being both planned and necessary, as opposed to, for example, making the dystopia the end result of a series of well-intentioned mistakes. But any motivation is clearly shown to be false, along with any claims of necessity, when the oppressive system is overcome. Evil overlords? For goodness' sake!
3 - The 5 volumes that comprise the omnibus edition are quite uneven. The first three stories are just mildly boring. The claustrophobia of the Silo is almost noticeable. On the other hand, the last two stories are much more verbose and the previous negligible tension of the first 3 stories is lost.
As a dystopian world it needs developing as much as its characters, and unfortunately it never quite got there for me.