I'm sorry to say this, but not everyone knows that Kaidou is protected by the plot armor of having been an adversary of Luffy's. Luffy's adversaries usually receive very good portrayals, take the most recent case, Lucci.
Luffy is known for performing miracles and the impossible. His defeat of Kaidou was a divine miracle, and we saw the enormous amount of help he received. Oda won't dismantle all this buildup by having Imu shoot Kaidou or something like that; he won't allow that, even against a 21-year-old rookie Kaidou.