Likeness rights. Most UFC fighters sign away their likeness rights when they sign their UFC deals. That means that the UFC can use their image/likeness related to anything promoting the UFC including video games.
Brock didnt sign away his likeness rights in his last UFC deal so EA would have to make a deal specifcally with him to include him in the game. I also believe that 2k has exclusive rights to his likeness right now related to WWE2k (he was on the cover last year.)
Given that this game promotes the UFC and its fighters, its highly unlikely that we will see current Bellator fighters like Ben Henderson or Rory or Bader in this game. The UFC doesnt want to promote fighters that arent currently on its roster.
I have no confirmation regarding this but I bet Rumble will still be in the game because he is retired but still under a UFC contract. Rampage is also possible because he is still under a UFC contract (his Bellator contract just ended).