Is because Kaido was just too strong be logically beaten by Luffy otherwise
Oda probably couldn't find a way to use Luffy's rubber fruit in such a way that it would give him an advantage over Kaido since it was beyond his limits as a writer. Even a "normal Gear 5" - an awakening of the normal rubber fruit and changing the environment into rubber - wouldn't have been enough to bring down Kaido in a way that isn't bullshit. Oda had to give Luffy a literal God fruit that's only limit is the imagination of the user and Toon Force powers to bring down an exhausted Kaido in an extreme diff fight. Oda wanted to make Kaido strong due to the hype and to meet the expectations of the readers but he couldn't do it correctly in a narratively logical way and he made him too strong that it eventually led to the Nika's asspull
Oda basically cornered himself the same way Kishimoto did with Madara and Kaguya and he had to relay on a convenient asspull to save the MC's ass and move the story forward
Oda probably couldn't find a way to use Luffy's rubber fruit in such a way that it would give him an advantage over Kaido since it was beyond his limits as a writer. Even a "normal Gear 5" - an awakening of the normal rubber fruit and changing the environment into rubber - wouldn't have been enough to bring down Kaido in a way that isn't bullshit. Oda had to give Luffy a literal God fruit that's only limit is the imagination of the user and Toon Force powers to bring down an exhausted Kaido in an extreme diff fight. Oda wanted to make Kaido strong due to the hype and to meet the expectations of the readers but he couldn't do it correctly in a narratively logical way and he made him too strong that it eventually led to the Nika's asspull
Oda basically cornered himself the same way Kishimoto did with Madara and Kaguya and he had to relay on a convenient asspull to save the MC's ass and move the story forward