Because of G4. No other usage of Haki burns through it like maintaining G4 does. Luffy never has an issue with Haki outside of G4. A massive power boost like that, comes with a massive drawback. Luffy never has any issues with Haki unless he's using G4. This time around, it's because he didn't have much Haki to begin with going into his 1 v 1 against Kaido.
During the Katakuri fight, Luffy had hours to recover his haki, and at the end of the fight, he was able to maintain Snakeman for an hour. Right now, he is just coming off losing his Haki, which is why he didn't have much to spare to begin with.