Luka is 40 lbs. heaver now than his rookie year which wasn't all that long ago. He's been getting injured more and more often which the team says is a result of him not being in shape, and it doesn't look like he has any real desire to change that. There's a clip of him drinking a beer after a game and a teammate taking it away from him.
And being a year away from a Supermax deal, Dallas avoids what it sees as a potential Zion Williamson situation of having an overweight, oft-injured player they can't get rid of. Anthony Davis is on the downhill side of stardom, but he goes back to being a true 5 which he's better at and they get a good 3-and-D guy, too. It's a win-now move on what is already a good team instead of rebuilding.
Although at least assuming the Lakers had draft picks to trade, Dallas should have been able to get a couple, especially with LeBron retiring probably at the end of the year.