We drafted him as a project, so I assume that Ainge understood that he could be half-baked at this point and could need more time learning the game.
Unfortunately for Young, Ainge stockpiled picks and the team got good, and now we have a roster crunch. This could lead Ainge to cut bait quicker than he would have otherwise. Also, we have Brown, who already looks close to being a contributor.
If he still seems like a better prospect than our late picks, Danny would just keep him and let the late round picks walk.