Its because his (initial) dragon form is too easy to wound or trigger a react from. Zoro cut his scales, Killer penetrate his belly, scabbards made him bleed, Law and Kidd made him scream. Oda shouldnt have done this. Dragon is like the peak fantasy creature signifying ultimate power etc etc, Oda shouldve shown the scabbards and even supernovas unable to entertain even Kaidou's dragon form. Once he goes Boro Breath then the breath reaches unbelievable level of destruction, like evaporizing the entire Wano ocean or evaporizing a few islands like Psykorochi's laser. If Zoro cut a mountain, Dragon Kaidou's breath should do this
Everyone should scatter away fleeing like flies due to sheer power and destruction of Boro Breath and no one even think of blocking it.
...thats what i feel we should get when Oda tell me about WSC. Not some dude who give free hits to the ones below him but become bloody and screaming anyway. Its weird.