It's just hard to see anything in RotK that wasn't done better in one or both of the others, excepting a few good scenes here and there. The battle scenes are bigger and still more spectacular, perhaps, but also cornier and riddled with things that I just found absurd (the army of the dead, Legolas' stunt with that elephant, Aragorn's utter and complete lack of anything resembling a strategy in the Black Gate battle, etc.).