Because you are actually GW Bush?
Actually, it was an educated WAG. Mitt can't afford to run again and he needs an excuse to walk away from the corner office on Beacon Hill. GWB does him a favor if he gives him the nod as long as the job can be accomplished in a year or less. It will work just fine for Mitt because it will give him a lot of national exposure and he will be owed (he will buy favors with his position) favors all around the south.