I like to think of it this way… anyone can learn a second language any time in their life. However, it’s proven to be much easier at a young (elementary) age when that part of the brain is still raw and developing.
A lot of these guys are just finishing growing and getting into their “adult” bodies when they are reaching college. A really critical time in their development as players. If they aren’t being developed properly, or worse, being allowed to pick up bad habits, it’s much harder to teach them to improve fundamentals / change bad habits down the line.
While the D league coaching is obviously better, there is a reason very few athletically talented guys ever go from a 4 year college career to the D league to success in the NBA…