The Gorosei and Imu want to keep their powers as a secret to their underlings, so Saturn can't transform while Kizaru and his Marine forces are around. Once the Marines have been defeated and its only the SHs, Vegapunk, and the Seraphim left, Saturn can transform and fuck them up.
This will allow for Luffy to fight Kizaru first and then fight Saturn
This will allow for Luffy to fight Kizaru first and then fight Saturn