From the 2008 Edition of the NEC Handbook, "Type 2 surge protective devices are permitted to be connected on the line side of the service disconnecting means. Such applications are permitted only in listed equipment, and the conductors connected to the surge protective device must have a disconnecting means and overcurrent protection. Field connection of these devices to the line terminals of the service equipment is not permitted."
I do not know the why unless it is to insure the lightning protection is still there when some of the service disconnecting means are open.