From what I can tell, a significant portion of GT's fanbase is fairly casual. This component of the fanbase is much more likely to be turned off by the PS3's price and much less likely to buy a driving game if they already have another one they're happy with.
While the various Xbox 360 driving games haven't done nearly as well as the Gran Turismo series, I think it's possible that they may have drawn away some of Gran Turismo's fanbase.







