Certain system parameters scale with the size of the Lowpan. With flat routing, the routing tables grow at least linearly with the number of addresses in the network.
To minimize the amount of RAM used in Lowpan nodes, we want to be able to limit the size of these tables. MidsizeLowpans may limit themselves to 256 nodes, LargeLowpans may need to scale to 10000 or more nodes. (There is little need to discuss RoutingScalability for small Lowpans, e.g. in simple home networks.) Assuming, say, 10-byte routing table entries, this leads to a requirement for 2.5KB or 100KB RAM, respectively, for routing tables alone.
One approach is to minimize constant factors in this equation. E.g., moving from 64-bit to 16-bit addresses reduces the size of the part of the routing table concerned with addresses by a factor of four.
Larger contributions to RoutingScalability can be achieved by introducing hierarchy into the routing, e.g. by partitioning the routing tables into global and local maps. To realize the efficiency gains of HierarchicalRouting, AddressAssignment needs to consider this routing hierarchy. To achieve reasonable efficiency, this means that AddressAssignment needs to reflect topology, leading to issues with AddressStability.