That and the fact Hill has a finger injury and Book still is inactive, the way this season is going Book has no real help on offense and would get killed so Payton is going with his known commodities rather than his unknown commodity in Book.
I agree AND Book is not a totally unknown commodity to Payton. He has been watching/charting every throw and every decision Book has made in every practice.
If Payton thought Book was ready for more than running the scout team, he would make him the backup so Book could get reps running the install.