Because his absence was most deeply felt by me (for the good guys), and because it's been clear that he has a really big role. Thom and Lan were always people who, for me, could wander in and out of the story at will. I just figure that because we have so many heroes, those absences are less important. We have only a few faces of evil, though, so they have to work harder to present their side. That's why I find the villains' in-and-out-and-gone-for-a-while unsatisfying.

You're probably quite right in saying that this is a coming of age story, and less about the "evil" side of the Good vs. Evil conflict. Also about the editing.