A sportswriter I follow (Andrew brandt) has a couple maxims and sayings.
"There will be lawyers"
"Deadlines spur action"
And the most useful one here:
"When the question is why, the answer is usually money"
In this case, I think Dallas looked at what they had with him and decided he wasn't worth the supermax they were about to be forced to pay him. I think where everyone has different opinions is about 'worth'. Yes he's 25 and just took you to the finals. Yes he's averaging 28/8/8 over the past 5 years (for those that don't follow basketball, that's very good). But he has an injury history. He has a history of showing up overweight and out of shape. He doesn't play defense (and I think he's actually a defensive liability). He definitely has an attitude that you can see on the court. What we the viewers can't see is him in the locker room. If he's all of the above but a good teammate then they probably work with him to fix all of that. If he's all of that and disruptive in the locker room then that's a problem. I think Dallas came to that conclusion and that's why they traded him.
That's not to say they got what they deserved. I think they honed in on AD and said "that's our guy". When you're negotiating and only have 1 main thing that you're after you limit how much you can get from the deal, especially when you're the one that initiated it. If it had been open bidding they could have gotten more for sure.
You also have to look at the flip side for the lakers- they just lost their #2 guy. I haven't looked at the analytics, but when I've watched them play and AD isn't on the floor they are a below average team and only look good if someone not named LeBron is having a hot night. Without AD on the floor they are small (even with him they were small). Now what are they going to do for a big man?
I think its a short and long term win for the lakers. While they have lost a big man and a lot of defense, they now have the ability to split Luka and LeBron and always have someone on the floor who will kill you from anywhere on the court. They can put them both on the same floor and you have to decide who to guard. If Morris can hold down the fort on defense then I can see the lakers climbing to 2 or 3 in the west and making a legitimate run to the finals. As they were configured before, the only way they were getting there was if someone else got hot (Reeves?, Hachi?). Long term, you have the opportunity to work with and coach up a top 5 player in the league.