I think he was the "Dante Exum" of the last draft (relatively unknown quantity, young, great tools and potential playing against questionable pro competition overseas), and I think GA is the reason Exum went so high this year. Kind of like how Yoenis Cespedes opened the door for Abreau and others in baseball- the first team that takes the risk that others passed on opens the gates up for higher payment (in this case draft position) of later, possibly less talented, people from similiar situations.
As for the passing up, if I remember correctly, the word was that he couldn't shoot at all. Fresh off of Royce White (good skills, no shot, and anxiety) that people had previously touted as a possible stud, I don't think experts were willing to label a good skill, poor shot type of guy a star too early. That, and people were wary of how good his other skills were and if they were strong enough to translate to the NBA at all due to little exposure in the US. There were questions about the talent he played against, and no one predicted he would grow the 3 inches or whatever that he did.