Primarily he's probably hiding it for hype reasons. It's the PU for his new major arc fight, doesn't do it any good if Oda shows it in action multiple times before he's entering that fight.
And additionally I think it's simply too powerful in a way. Shit can destroy all sorts of weapons, armour and probably packs a hell of a punch.
Granted that some second or third graded antagonists like Paypay and Ulti are simply not meant to be taken down by Luffy early on, it might be, that Oda just cannot let Luffy use it on them, because they indeed would be out cold.
And on the contrary, it would lose a lot of hype if anyone and their mothers could be able to tank it and go on, just because they weren't meant to go down there and then. It's a dilemma tbh. So I think it might better this way.