After digesting this trade for few weeks I have ruled out or dismissed all theories except one.
Luka has some underlying medical issues that are not going away. I'm thinking something like stress based bone issues which carry a risk that Luka turns into the next Embiid, and which also really hurt his ability to work on his conditioning. Mavs medical staff knew and Lakers learned this as part of the trade, which was the reason why they were able to negotiate the price down (them being able to do that makes almost no sense otherwise). Lakers accept the risk and hope they can manage Luka's workload and training to minimize the impact.
They said "conditioning worries" was why they were able to exclude their other FRP and Knecht from the trade... but I think thats BS. They should have had ZERO leverage to push the price down to where it ended up. I think they just couldnt disclose the real reason since detailed medical information like that cannot be made public due to privacy laws.