They think it's a waste of money (since they never send players down) and they think their project players develop better practicing with the team and assistant coaches than playing D-league games.
The NBA really needs to form a farm league/system like MLB has. Increase the draft to like 3 or 4 rounds (with the 3rd and 4th rounds being players for the D league teams).
I never knew why it existed, other than to advertise the NBA in areas that have no NBA team. Always makes more sense to be practicing with your own team over any other situation.