I think most people understand how great KG was as an all-around, complete basketball player. But throughout his career, he consistently was unreliable in the clutch, because he didn't have particularly good ability to create shots for himself that he could consistently hit under pressure. Generally, he needed someone else to create a midrange jumper for him, and while he was generally money with it, however he had a tendency to start bricking that shot bad in the final 5 minutes of close games.
The reason most people are choosing Dirk, is because they are looking at it from a perspective of "who would you rather build a team around?", not "who is the better all around basketball player?". And honestly, I think the first question matters more in terms of an individuals greatness. There have been plenty of players who were better all around basketball players than Magic and Larry Bird. Both of them were weak defensively (much like Dirk), Magic wasn't a particularly good jump shooter, while Larry lacked physical advantages that others had. But it doesn't change the fact that they are two of the 10, probably 5 greatest players ever, mainly because of their leadership and ability to produce for their team when their teams needed them most. KG has NEVER been a guy known to consistently come through for his team in the clutch. But somehow since he acts like an asshole on the court and barks and punches his chest a bunch, that makes him look "tough" and like a "leader", even though his on court performance when his team needed him most didn't prove any such thing.