Well it all started with the injury Bol suffered at Oregon, this injury resulted into only a 9 game college career. In those 9 games Bol averaged 21.0 points, 9.6 rebounds, 1.0 assists and 2.7 blocks per game, during all of that scouts were iffy on Bol's work ethic and heart for the game, then came the NBA draft on June 19, 2019 in Brooklyn, New York. Going into the draft Bol was projected a lottery pick, but as the draft went on his name wasn't being called, finally with the 44th pick the Heat selected Bol Bol, then later on the Heat traded Bol to the Denver Nuggets.
Right now Bol is on the Windy City Bulls since Denver doesn't have a affiliate in the G league, Bol is putting up solid numbers for a rookie, he is averaging 12.3 points, 5.71 rebounds and 2.29 blocks per game with Windy City, but Denver already has Jokic who is an All Star, Millsap, Grant, and Noah Vonleh. So the Nuggets are set with the center position, for the production Bol is doing in the G league its not worth the risk of calling him up if you already are set with F/C position.
Some other reasons is the reputation and scouting report on Bol.
Bol is 7'2 but only 208 pounds, and yes Bol can do a lot on offense, but the risk on defense is huge, ya he can block shots, but against strong, solid veteran centers he is going to get pushed around. Bol also has a reputation of not trying very hard and not having a work ethic, this is a huge problem when you have hungry young players trying to take your roster spot. I believe the Nuggets are taking this situation very slowly and are going to be very patient with Bol.
This will for sure be a "work in progress" prospect, but once he puts on some weight and develops even more in the G league, the Denver Nuggets have a great prospect for the years to come.