Well to answer your question, he was not a highly touted recruit coming into college, we can start there. He was rather low on rivals.com around the 49sh mark. He wasn't a winning player in college either and it was due to all of the factors pointed here, the huge TO rates, the bad decision making, taking bad shots, and bad defense.
I think his college coach had to deal with some limitations in him because he's not a shooter and had to play him as an on the ball guard.
I don't know what to tell you but he's still a young guy, so I am choosing to be optimistic. If his handle is not good enough ever to be a main guard then he needs to be SG and play off the ball, for which he needs to shoot a lot better. At least he has size, length and athleticism to still be an NBA player even if he's not a PG, but that will leave some fans disappointed.
I look forward to seeing him improve. He does seem like a hard working coachable player and the Spurs more than once mentioned that they are excited about the person and him being coachable, but he's definitely a project and won't be able to be the main guard until his handle gets better. I think it might be possible for it to improve. Wasn't Kawhi a poor ballhandler coming into the league around the same age? It's possible, but it could take years, certainly not the short schedule that fans want to see Dijon on.