In my opinion, it is much easier for a DC to operate from the sideline. The defense, at it's most basic level, is just reacting to what the offense is doing. The DC tries to get guys aligned to give the greatest advantage given the situation, but at the end of the day it's up to the defender to defeat a blocker and make a tackle. However, the OC has much more ability to change the game by adjusting alignments, changing blocking schemes, using formations etc to control where the defense lines up. Therefore it's much more important for the OC to view the offense-versus-defense matchup in a wholistic fashion - unit against unit rather than player against player. So the OC really gets the greatest benefit by being in the box.
I also think the "feel" of the game is much more important defensively, when a burst of adrenaline can better help a player react. I want my defense playing like their hair is on fire, and the ball carrier has the fire extinguisher. The offense needs to be calm and in control, and position coaches and the head coach can manage that from the sideline without having the coordinator down there.