Mostly because TJ isn't a small bantamweight, and he's never made 125, ever in the UFC. If he could "easily" make 125, he would have moved down already. This tells me his fight against Johnson will be a one-and-done. Win or lose, he won't stay at 125. If that's the case, then why would he care about making weight? Sure, being a 2-division champion is nice, but securing the win here is paramount.
Following this logic, TJ just wants to go down and beat Johnson, make him look bad, and break his undefeated streak. It doesn't actually matter if TJ wins the championship. Therefore the best way to do this is come in heavy, say 129 (rehydrate to 145). You could make some excuse, but say you're still willing to fight. All the while you intentionally missed weight and aren't more drained than usual.
It's basically making it a catchweight fight @ 130 without getting DJ to agree to it. And what's DJ going to do? If he refuses to fight, he'll look like the bad guy. But if he fights, he's facing an overwight opponent. It's a lose/lose for DJ, which is why this fight sucks. DJ deserves better.