The Fire nation has to have replacements delivered from afar while the earth benders can just act as siege weapons at a squad level. The Earth benders can cause all sorts of havoc for a besieging force, remember these guys can tunnel through rock at walking pace, the Fire nation could set up big arrays of siege catapults only for a raid of Earth benders come from underground and collapse all their siege weapons into a pit. The Besieging fire nation on the other hand is stuck in the middle of a desert at the end of a very long supply chain in occupied territory with partisans running about. The city has a fast public transport network, unlimited supplies and huge defenses that it can easily repair. It was effectively under siege for decades and the inner city was oblivious. Ba Sing Se is an entire country with everything it could ever need within its walls. The fire nation has tried conventional sieges against Ba Sing Se, they failed and for obvious reason. First of all its not wank if its supported by every showing of the Trebuchets in the show.
