Because obliterating Onigashima would not have gotten rid of Kaido. In the worst case, Kaido would have understood where that move came from and would've targeted the WG, maybe even while being backed up by Big Mom.
So then why isn't Imu getting rid of Luffy and Blackbeard, if he/she cares for the order so much? Luffy literally was the Joyboy time bomb, and Blackbeard the evil version of Luffy. But instead of obliterating them, he or she is butthurt and tears up posters.
Imu is sitting in his garden, being butthurt and tearing their posters apart. Weird.