The Gran Turismo series outsells the Forza series by far. The entire Forza series: Forza 1 through 5 + the Horizen games only equal about 20.50 million sold. The entire Gran Turismo series: including prologues have sold over 70 million!
To put this into more clear perspective, Gran Turismo 1 and 2 alone have almost sold as much as the entire Forza series. Why don't Xbox fans support their top racer better?
Yeah, it's a shame, Forza is such a good racing game, easily on par with Gran Turismo. But I guess people are just used to the name. Gran Turismo is for many people the definition of a racing game as Playstation is the definition for a video game console. They just don't know better.
But we also have to keep in mind that GT5 sold the most in Europe where PS is way stronger than Xbox. I guess racing as a whole is more popular in Europe.
And let's not forget how bad GT6 sold...