To answer the question, NBA GMs have personal interviews, background info, and medical info that we do not have. I'll simply reiterate my supposition that there was something about Luka's intangibles that Phoenix, Vlade/Peja, and Atlanta did not like--or at least preferred another candidate. As an aside, the crazy thing about last year's draft is that it's, like, one of the all-time top 5s to ever be in any draft and four fan bases are despondent over one guy when everyone got a really great prospect.
Anyway, say what one will about Vlade, but I believe he is pretty plugged in on European talent--particulary talent from the Balkans that grew up at Real Madrid. Phoenix had Igor on staff and was run by McDonough, an Ainge disciple, at the time. Ainge is infamous for his pre-draft personality mapping--presumably McDonough engages in the same. Of course, we can debate whether Sarver made that pick exclusively, or not, and McDonough was bizarrely fired later that summer. I would like to foot stomp that Ayton had significant warts as a prospect, and was not an automatic #1, despite what many people claim today. Atlanta was run by Schlenk, a Warriors/Meyers/West protege, and went with Trey over Luka. Trey's an exciting player, and his defense is sometimes so bad that it's good and he picks up steals by being a zip code away from where he should be and intercepting passes, but he's going to get destroyed in the playoffs. Point being, three front offices that had a very solid grasp on Luka the prospect, all of whom had a positional need (playmaking 4/big PG), decided to pass.
We will probably never know the full reason(s) Luka slid on draft night. Whatever it was, it certainly does not appear to be effecting him right now. But, it's still early in all of these guys' careers and time will tell. At the end of the day, these are all just hiring decisions, and a new employee that blossoms for one division in one geographic location may not have necessarily blossomed somewhere else. It's quite possible all four teams were correct about Luka, and Dallas was fortunate to be the right place for him. I will say that Dallas is somewhat infamous for being accommodating to flamboyant or larger-than-life figures across all sports--again, not saying that's necessarily the issue here, but simply illustrating that all four pro teams in Dallas take big swings on prospects. It's a go huge or go home kind of town.
Two other notes: 1. I do not care what Joerger's comments were on the topic. Joerger is the Jeff Fisher of the NBA. Drafting Luka=new QB=new offense=extension for Joerger...because how can you evaluate Joerger's performance when you just gave him a new PG and he had to install another new offense (i.e. the Jeff Fisher)? Joerger was far more interested in the office politics surrounding the decision than the actual future of any of these prospects.
2. More controversially, we would have had to trade Fox. Yes, we would. Moreover, we would have made the same exact Zinger trade, only not for as many picks (and maybe no picks, depending). Many people think Fox and Luka could have played together, and they are right. On a basketball court, Fox and Luka would fit nicely. Unfortunately, the NBA is a basketball-based entertainment company, and not pure basketball. So, you can't have a team with two rookie contract ball handlers both chasing max extensions in NBA entertainment. One of those guys would have to be the A-lister, and the other will demand out. Back to Zinger--whomever drafted Luka was going to make that Zinger trade. The Zinger is simply too valuable, and too great a fit on paper, for the team that acquired Luka. And now, they are chasing multiple roster holes with very few picks. So, we'll see. But, that was going to be the gamble for whomever selected Luka.