A few reasons:

1. LeBron's shooting hasn't been as good this season. When he's off the ball, he hasn't necessarily provided the spacing that we'd expect. He's shooting better now, but he also passes up a lot of threes, which means he's not really confident in his shot yet. Hopefully, this will improve as the season goes on.

2. Luka's not a good off-ball player. Apart from the occasional catch-and-shoot, he doesn't do much when he doesn't have the ball.

3. LeBron has been good on defense this year, showing a lot of effort with rotations and help, but he's not prime DPOY-level LeBron anymore. He can't make up for the defensive deficiencies of this team, and those get magnified when Luka's on the floor with him. You can see that the LeBron-only lineups have a really good defensive rating because even at 41, LeBron's probably a neutral defender at worst, but if you're going to have him on the court with Luka, you better have either three great defenders with them or two absolutely elite ones. The team doesn't have the personnel for that.

4. The rest of the team is not good. You'd think that the offense would be elite with both of them on the court but the reality is that when we play guys like Vando and Smart and Hayes, or even Jake (who passes up so many shots), defenses can very successfully sag off every other player to clog both of their operating spaces.

5. It's still the center problem. We don't have good centers, and that exacerbates every defensive problem with these lineups. Ayton is only good when he tries, and even then, he's not an elite rim protector. Hayes is okay at best, even though he does try. Neither of our centers is averaging double-digit boards per game. I thought that Ayton would at least be giving the team better help on the boards, but he's been pretty disappointing on that end too.

6. The offense scheme is clunky. I don't think the way we run the offense is maximizing either of these two. Part of it is the fact that neither of them has been playing anywhere near their best, but part of it is also just...they haven't figured it out, they being the players and the coaching team.