Logistics. We got more stuff on Guadalcanal than they did, and we brought more to the fight at every island after that, as they could not resupply.
In the Pacific campaign, our landing ships were more important than our surface ship gun power. Getting ammo to the guns is more important than the quality of the guns.
Wrong: you forget that the navy skedaddled almost immediately when things went very badly in the Battle of Savo. All that was left were what had been piled on the beach and when we could use that the Japanese left. The Marines fought with very little food and supplies until the navy finally overcame the Tokyo Express.
Recommend reading Guadalcanal by Richard B. Frank.