Because he had Tebow as a glorified fullback running the iso... and when he did have a power RB (DeShawn Wynn) he just happened to lead the team in rushing both years he was there. Shoot, in 2006 he pretty much ran his offense with two power backs in Wynn and Tebow with very good success on the ground.
After Wynn graduated, he didn't really have a power threat in the backfield besides Tebow... and pounded him as much as possible. He finally brought in a power back in Mack Brown, but wasn't around to see how that panned out.
ZM I like you, but you're off on this one. Urban has a history of tailoring his offense to his personnel, and does whatever he can to get them the ball as much as possible. If Brionte earns PT, Urban will utilize as best he can with isos, off-tackle powers, etc.
If you haven't already, take a gander at what Ross and the guys wrote up about Meyer's offense at
Alongtheolentangy.