It's probably down to licensing these days; car companies wising up to the value of their product and charging a lot more/doing exclusive deals (*cough* Toyota *cough* Gran Turismo seemingly).
That's not to say they couldn't do another smorgasbord of racing one day (I liked in ToCA you could be doing touring, ovals, open wheeler, historic, rally, rallycross, monster truck, trucks, even Honda quads and mowers!! The rally bit was a nice crossover, almost a mini sales demo for the Colin McRae games. I hope the F1 bit might be a mini promo for the F1 games, but it's much more likely it'll have dialled-down arcade handling (wonder if we'll get handbrakes in the F1 cars like ToCA Race Driver haha!).