For the same reason why Crocodile didn't use his sand tornados or blades against Luffy in their third and final fights, why he didn't make sure that Luffy was finished the two times he beat him, why Doflamingo didn't use Parasite against Luffy or Law except during his futile attempt on G4, why Smoothie didn't easely cut the treasure room door to let Brook copy the Poneglyhs or used horizontal versions of her Flying slash, why Perospero didn't use his df to lift their ship above the waterfall, why Akainu and Kizaru didn't melt the ice created by Aokiji or aimed for Whitebeard's head when he had a heart attack, why Big Mom didn't use any of her real power during WCI arc, etc..