First, Nidoking's best attack moves are Earth Power, Sludge wave, Thunderbolt, Flamethrower, and Ice Beam. Out of these 5 moves, Earth Power is the only one with less base power than Nidoking's physical move of the same type. And as the other answer already said, Earthquake is not boosted by sheer force.
Nidoking is equipped with an incredible STAB combination that is amplified by its ability Sheer Force and a Life Orb, allowing it to operate as a powerful special wallbreaker despite its mediocre base Special Attack. In addition, it boasts phenomenal coverage options such as Thunderbolt, Flamethrower, and Ice Beam that allow it to reliably pressure switch-ins to its STAB attacks such as Slowking, Corviknight, Ferrothorn, and Landorus-T.
Nidoking uses Thunderbolt as its most consistent coverage option, super effectively damaging Slowking and Corviknight...Flamethrower is another coverage option that reliably OHKOes Ferrothorn and Scizor and can apply pressure to Landorus-T switch-ins. Alternatively, Ice Beam targets Landorus-T and Garchomp but gives Nidoking a more difficult time in pressuring Ferrothorn, which can take advantage of it by using Knock Off, Leech Seed, or Spikes.
Source
All the Pokemon with bold names in these 2 paragraphs are Pokemon that usually have higher physical defense than special defense (or Intimidate in Landorus's case). This is another reason why Nidoking's special attacks usually do more damage.
