Devil Hulk is suppressing the other Hulks
I believe that in a way the Devil/Immortal Hulk isnt just a father figure to Bruce or the Savage Hulk, but to all of them. In other words, what he basically did to the professor, the green scar, and the other missing Hulks was akin to putting them in a corner or grounding them, but still having access by watching the adventures of Bruce and the Devil Hulk, the only exceptions being possibly Hydra hulk(because he is dead) and the guilt hulk.
- Further theory if this is true: the reason why Devil Hulk has "put in the corner" Green Scar, it was because of the latter's failings as a father. Considering that Devil Hulk sees himself as a father figure Brian Banner never was and how Green Scar's fight with Skaar ended the former realizing he is acting no better than Brian (not to even mention the fate of Hiro-Kala or how Gren Scar essentially dumped Lyra on Jennifer's lap), it feels like something Devil would take an issue with.
The One Below All is consuming Hulk from the inside
This theory was already mentioned as an alternative explanations where did Professor and Green Scar go in other parts of this article. Adding it to the list for the sake of consistency.
Amadeus Cho is correct why Professor and Green Scar have gone silent.
Amadeus' theory Banner's two most heroic Hulks have gone silent as a protest against Bruce and Devil Hulk's action is another possible reason for their disappearance.
The Green Scar has been consumed and/or replaced by The One Below All.
The Green Scar is shown blatantly lying to and manipulating the Savage Hulk, beats a chained-up Devil Hulk for no given reason, and brushes off the idea of freeing Joe. Additionally, he doesn't actually do anything to assist the Savage Hulk, instead sitting back and prodding him.
Probably the most concerning pieces of evidence that something is very wrong are that when the Green Scar briefly takes over to kill Xemnu, not only is Savage Hulk left with no memory of what happened but Hulk's veins bulge grotesquely, spikes erupt from his arms and back, and he refers to Earth as "his planet."
All of this is very much out of character for him, given that he's actually one of the more heroic and reasonable Hulks. The fact that the Breaker of Worlds looks like the Green Scar and shares both his title and motto, how the Leader tells Xemnu to "look deeper" after he boasts about having control of Savage Hulk's mind, and how he refers to this Hulk as "coming up to meet you" further emphasize this possibility.