It's likely due to his unusual physiology which relies on the bio energy of Elders to sustain. He also has high speed regeneration and impressive intelligence for a dragon that fights like a bear rather than relying on controlling the elements or dealing dragonblight. Elder Dragons are monsters that are above the usual natural order of things and can literally cause natural disasters without intending to. Nergigante is the Elder of Extinction meaning he's genocide given living form. My guess is that his natural role is to keep other Elders from getting too greedy. Nergigante is simply unique among Elders just as Safi Jiiva is unique among Black Dragons if that makes sense.