I've paid attention to how Loma's career was moved to get him to undisputed, so I have a simple answer with a little bit of back story.

Let's keep in mind that Loma has repeatedly reiterated that he's too small for Lightweight. He also stated that he only moved up because he wanted to become undisputed, and he'd have a better shot at getting the fights at Lightweight, since the belts were mostly at Top Rank. He wanted to get 4 belts and then drop back down. They wanted to hurry that process up, likely because Loma is taking too much damage from bigger guys at Lightweight. One way they sped the process was appealing to the WBC to award him the "Franchise" WBC title, which meant he now had no mandatory and didn't have to fight Devin Haney (This is how Haney became WBC champ. He literally inherited the title w/o having to fight for it.) So now, the last big guy Loma would have to fight would be the winner of Lopez vs Commey.

So to answer your question, Loma agreed to no rematch so he wouldn't have to take more damage up there at Lightweight. As good a fighter as he is, it was clear that he wanted to get those 4 belts and gtfo of 135. He thought he'd win, drop back down and call out Gervonta Davis again or fight Shakur Stevenson. I view it as planned quick heist that went wrong.