Who is the narrator, can she or he read minds, and, more importantly, can we trust her or him?
Third-Person Omniscient
We don't know the guy who's telling this story, but it's clearly not one of our heroes. He can look into Theo's head and into Cabbarus's brain, which nobody can do here (not even Dr. Absalom), which is great for us since it helps us understand each character's actions and motivations. In general, the narrator appears to be an unbiased third-party observer who just tells it like he sees it.
If the point-of-view was in the first-person, the audience wouldn't get nearly as good of an understanding of each character's motivations. For example, if Theo told the story, we would never know what Cabbarus was up to back in Marienstat. Or if Mickle narrated, we'd miss out on most of Theo's journey and the role Cabbarus plays in oppressing the people.
By providing such an all-encompassing voice, the author allows a bit of insight into the brains of many characters, providing the readers with a lot of information that allows us to get a really good picture of everything that's going on.