I mean 3 players have been sent down, and every team is like, well it's better for our guys to be here under our supervision, blah blah, before it was well it would be good for them to have a place to play.
Not to mention it was introduced at the same time the NBA announced the new age rule. And foreign players certainly aren't going to spend time there when they can make good money playing back home in Europe.
It was dumb for the NBA to have D-League still take up roster spots on their NBA teams. If they didnt count a D-League guy against the 15 man limit, teams would have more incentive to send people down.
For example, the Pistons traded away Ronald Dupree so they could get down to 15 men. They could have kept him and sent one of the rookies to the D-League instead.