I have a theory.
I think he’s someone who cares a lot about his appearance. How he looks, how he dresses, what he drives, how he plays, his reputation, all of that. I think that being a great defensively is something he knows he can do and it makes him look good. Coaches, scouts, teammates, and fans all love it and respect it. Shooting threes is something he’s not good at and he doesn’t have confidence in, so he’s worried that trying and failing will make him look bad or will cause people to lose respect for him.
Obviously I don’t know the guy and - although a Sixers fan - I’m not hardcore into basketball like I am hockey, but I say this based off of my experience with person know who has a similar mentality, albeit in a different competitive activity. Just an idea.