Well, I think the issue is that Kelly clearly hasn't earned a starting spot yet, though he's had some good games. He shouldn't just be gifted a starting spot.
Sully has earned a spot, but we want him getting the majority of his minutes alongside Kelly to see how they fit together. Bringing him off the bench makes some sense, then.
Also, if we play the young guys heavy, heavy minutes in this last stretch of the season as the team spirals downward under the increasing weight of loserdom, it could encourage them to develop bad habits. It might be better to keep playing the veterans for (fairly limited) minutes ahead of them.
That's the best rational I can think of for not starting them, anyhow.