My two cents, it's the personnel. The Clips weren't doing anything special on defense. Their players just defensively murdered the Kings in the 2h. That's because:
1. Fox has the physical tools to be an elite defender at the 1 spot. But despite the flashy steals and blocks, he's soft. Still doesn't fight over screens...
2. Buddy is probably the worse defending starting two guard in the league. It is the primary reason why he should be a 6th man. Don't have to worry about defending.
3. Barnes is the only Kings player, who can guard the Lebron, Kawahi, George types. But not at an elite level.
4. Bags has the physical tools, but lacks the experience and the mindset. He's improving though. But not enough.
5. Holmes is a more physical, but essentially a center version of Fox. Defensively, he's flashy--at times. But he's not Whiteside who not only produce transparent stats, he freeze's defenders when they enter the paint.
6. Cojo is on his last legs. Outside of Woodard and James, the Kings don't have any other players to come in and defend.
7. Haliburton gets a pass, because he's going to be the Kings best player by this time next year. Someone would say he already is.
If it's personnel though, why are we worse than last season with mostly the same roster? We had Fox, Buddy, Barnes, Holmes, and CoJo all playing big minutes last season. The biggest minutes subtractions have been Bjelica and Bogdanovic and the biggest additions have been Bagley (who was injured most of last year) and Haliburton.
Conventional logic is that defense is largely a matter of effort and it's quite apparent watching the games that we're getting much more effort from Fox and Buddy on defense this season. Barnes, despite always looking solid as a defender, continues to grade poorly with defensive rating because of his low steal and block numbers. I would say in about half the games I've watched this season though he's been more locked in as a defender than at any point since his Golden State days. And yet only Buddy Hield grades worse as a defender. You're calling Fox soft but he's getting up into ballhandlers this year like he was back at Kentucky. The eye test just doesn't match the results.
This is probably selection bias on my part but I've seen a lot of times this season where the team plays very strong defense for 20 seconds and then gives up a back door layup or an offensive rebound and the other team winds up with a basket anyway. The other thing I've seen a lot of is somebody waiving feebly at an outside jumper coming off a missed rotation while they're two steps away with no possibility of actually contesting the shot. If I were to attempt to answer my own question, there's two trends I see happening:
(1) We lack any kind of physical intimidation factor. Holmes tries hard but he's under-sized for a center. Bagley is long and bouncy but he can be blown over with a stiff wind. He does an excellent job of moving his feet but as soon as the offensive player gets into his chest, there's not much he can do. Fox, Buddy, and Haliburton all play defense with their hands. They're solid in iso situations (Haliburton is particularly good at ripping the ball loose) but cutting off ballhandlers in space coming off rotations and communicating on picks have both been problems. Barnes was clearly taught a long time ago to keep his body between the ballhandler and the basket and he's ruthlessly efficient at it but he never forces the issue enough to make the offensive player uncomfortable.
(2) Because we can't intimidate anyone physically, we're trying to outwork them instead. That tactic might be effective against young teams like Chicago who fall apart when you apply pressure but a veteran squad like the Clippers will keep passing the ball until we play ourselves out of position and get either an open jumper or a back door layup out of it. Then the second half starts and we're exhausted from maxing out our effort on all these defensive rotations and suddenly we're both physically inferior and a step behind. That's a lethal combination.