It is a performance issue. With only the root album in control we can easily see if the album is managed. If subalbums could be in this category several things would be needed:
Testing for management would have to climb the object tree of the subalbum each time an album is accessed.
The selector list for managing albums would have to include all albums in the gallery.
There is also an implication of a hierarchy of user rights that would be a serious can of worms.
There have been only a handful of people requesting this feature. So we do not want the many to pay the costs of the few.