That's just not the way it works. Hart is MUCH better defensively than Douby and Hart has earned the right to at least be on the active roster. Quincy hasn't done anything to merit being activated ahead of Hart. That's not to say he won't; it's just that this is a business, not a video game. They're gonna go with the guys they think can help the team most; and right now that's not Quincy Douby.
If Petrie didn't think he had some value, he wouldn't have drafted him but Petrie looks at the big picture. Look how long it took for Peja to even get into a Kings uniform after he was drafted.
Good things come to those who wait...and Douby is learning valuable lessons by practicing with the team. The coaching staff will decide if he's earned the opportunity to put those lessons to use with playing time. With the new active/inactive rosters, he can pretty much be activated at the drop of the hat if Muss and the rest of the staff decide they need him.