We have been trying to add as many siege mechanics as possible, but we must keep in mind overall gameplay and how much they fit.
For example, adding them to Minas Tirith would be useless, since battle is happening on the walls and the catapults are hardly effective there. For Edoras, there are already easily passable walls, and it would become too easy.
Also, catapults can take a lot of the battle focus away from the actual flag and battle, something we don't want. The reason it works so well for Erebor, is that that gate battle is just ramming apart from the catapult, with no ladders or other way in.