That's because while Chaos and Death have the Monsters of Chaos and Beasts of the Grave factions, both Order and Destruction don't have factions specialized in beasts. They could definitely benefit from a Menagerie of Order and Feral Destroyers faction.
The Incarnates should have their own faction as it seems that "eventually" there will be 8 of them. Merwyrm should be changed to Order and put in the Scourge Privateers faction.