After Luffy vs. Kaido, the writing is pretty much on the wall as to why Kaido didn't use awakening, and that's because he's not awakened.
Bu...but how can this be possible? He's a Yonko, and one of the most powerful characters in the World!
As I said though, we already know why, because of Luffy's fight with Kaido, and that is the fact that in order for a user to awaken their Devil Fruit, they technically have to die.
That has been impossible for Kaido his entire life. Nobody has been able to even push him to the brink of death, let alone get him to the point where he needs to be to awaken his devil fruit, which is to actually die.
I guarantee you that's the same case for Big Mom, and why she didn't use awakening. Neither of these two individuals have been in a fight where they were pushed so close to death, that their devil fruits awaken.....
BUT..... What if...... Both of them falling into the magma was what was needed to finally awaken these two monsters? This is probably the first time in their lives that they've both been in a scenario that puts them on the verge of death.
I can see when the EoS war begins, these two breaking out of their magma encased tombs, fully awakened, and ready to join the final war.
