I think it's because he doesn't quite fix the main problems that the team has and is potentially an ill fit with Fox since they've shot similar from beyond the arc over the course of their careers.
No one thinks that Sabonis is a bad player, he's the best player we've had since Cousins. The issue is whether acquiring him will just put us back in the Cousins 30 wins a year, middle to back end of the lottery era. The Kings need a top 4 pick this year extremely badly. If they get that and hit on a player, they could be a playoff team down the road with a Fox/Sabonis/top 4 pick core.
If he comes in and pushes them back to 8th in the draft and all they get is a backup wing out of it, then they're in the same situation they always find themselves in. Not bad enough to get the good players, not good enough to make the playoffs, not enough talent to get better through trades and no way of signing good free agents. It's why it's a potential rearranging chairs on the deck of the Titanic situation.
Oh, I get the apprehension. He's a throwback big man, and not an elite rim protector. He's not a terrible defender, he is the only player on the Pacers that is a plus on offense and defense, according to 538's RAPTOR. If Monte can trade for a more nimble-footed partner for him, then our interior will be fine.
Sabonis is not a great 3-point shooter, but he does have a good elbow jumper, and is in the top 20 for player efficiency, something I feel is being overlooked. He is being this efficient, on a team, that is one of the worst in the NBA in 3-point shooting, bottom 5 bad. Sabonis is a more fleet of foot Cousins with insane efficiency. I know he will become a fan favorite quite quickly.
I will miss Haliburton a lot, but I am excited to see Sabonis. It's insane that Monte was able to get this done without giving up any draft capital.