Maybe they are waiting for leg with the plates in it to stabilize a bit more. As much as Barbaro is beating all odds, you gotta admit tht he is in rough shape as far as any horse is concerned! :-)
It might actually be the laminitic foot they're most concerned about too... If he's still laminitic, every step might cause coffin bone rotation, they'd want to keep him quiet till all danger of that has passed.