I reckon this might be because Bivol's team considers Beterbiev as a very big risk for their fighter Bivol. They also might have a plan to observe first then accept the challenge later hehehe. In any case, it appears that Beterbiev wants to become champion again quickly because there is a rumor that after his next fight, he will challenge David Benavidez. This will certainly bring the attention and the advantage on the negotiating table back to Beterbiev and he will be positioning himself to become unavoidable for Bivol.
I read a news report yesterday stating that Dmitry Bivol underwent back surgery and will be resting for 6–8 weeks to recover. He’s expected to be out for the remainder of the year and plans to return to action in 2026. This could be the reason why the trilogy fight with Artur Beterbiev has been postponed. While the situation seems like a legitimate reason for the delay, it feels unfair to Beterbiev, who is aging and may not perform at the same level as he did in their first two encounters.
If I'm not mistaken, Bivol has been complaining about this back for years now. So it make sense that after his win against Beterbiev and avenging his defeat, he will have to take care of that back and underwent surgery to repair it so that he can continue with his boxing career. It might be a dangerous Bivol after he recovers 100% and be back fighting anyone at 175 lbs.
Even Canelo says that he is willing to fight Bivol to obviously avenge as well his defeat. As for Beterbiev, he already move on and as far as I know he will have a fight already in the schedule. I just forget the name of his opponent.