It hits harder
Despite having weak special attack, Nidoking actually hits harder with Sludge Wave compared to Poison Jab.
252 SpA Life Orb Sheer Force Nidoking Sludge Wave vs. 0 HP / 0 SpD Mew: 199-234 (58.3 - 68.6%) -- guaranteed 2HKO
252 Atk Life Orb Sheer Force Nidoking Poison Jab vs. 0 HP / 0 Def Mew: 187-222 (54.8 - 65.1%) -- guaranteed 2HKO
Not only that, but it's coverage is also more powerful.
252 SpA Life Orb Sheer Force Nidoking Flamethrower vs. 0 HP / 0 SpD Mew: 125-148 (36.6 - 43.4%) -- guaranteed 3HKO
252 Atk Life Orb Sheer Force Nidoking Fire Punch vs. 0 HP / 0 Def Mew: 117-139 (34.3 - 40.7%) -- guaranteed 3HKO
Going Special also has the benefit of not being crippled by burns or triggering on-hit effects like Rocky Helmet. Physical Nidoking also doesn't have Ground STAB boosted by Sheer Force.