I think 2nd hand sales also need to be addressed. IMO, FPS, especially Halo give more reason for buyers to hold onto their game than driving simulators or driving games such as GT. This is due to the online multiplayer and what it offers. One of the reasons for people loving Halo is the online mp. Whist GT is better known for its sp.
I remember in the ps2 days, I would find tons of GT3 and GT4 games stacking up in the pre-owned section.