Gran Turismo set the standard for racing sims on consoles. I don't think that anyone can deny that. The GT games on the PS2 were absolutely incredible for their time. Forza emulated them very well, but still, GT will always have that high level of recognition.
It also has to be mentioned that Forza runs into the same situation that makes FPS games sell so very well on the 360. The 360's primary user base is heavily centered around shooters, as that is far and away the top genre in North America. Racing games, on the other hand, perform better in Europe and Japan, markets where the 360 is the weakest brand.
Finally, it has to be considered that GT5 is better now than when it was first reviewed. The degree of support it has received is pretty impressive. Who is to say which game is the highest quality at this very moment? This is an area where Metacritic fails.
