A nice, simple, clean, serial structure. Attributes can turn into another list or hash. Such a memory structure does not have to be represented or thought of as a graph, and is easy to work with. It's the objects have properties which may have properties model.
Topics are instances of other topics, and so on.
It can't be represented as a tree because there are multiple routes between many of the items. Once it's in memory, you could have all these items linked up without any explicit ID at all, they're mainly needed for serializing. If you doubt this, just think of a linked list - you can work with each member of the list with no explicit notion of its memory address or object id. The same would be true of an in-memory representation of a topic map. The computer would, of course, use references that it assigned, but there would be no need for the ID attributes per se.