The problem with A1 GP is that teams go by countries (Team Canada, Team Britain, Team India.......) and not by manufactures or owners. So if PD adds one A1 GP team it will have to add all other as some might take offence to their country not being there. They can't segregate countries like that. It will hurt GT sales.
Also A1 GP is not geared towards American audiences so they won't understand much and would won't their own IRL or Champ cars. It just creates too much confusion. Thats why they go with the generic car and F1 seems the logical choice.
I will love if they put A1 GP cars but if I had to make a choice, I will pick all F1 teams over all A1 GP teams.