But it makes no sense. GT is a system seller...
I wouldn't be too sure about this, for two reasons.
First, Gran Turismo is now infamous for its zombie AI, vacuum cleaner engine sounds, long and delayed development periods, lack of a livery editor, and standard cars. These things together have made the game the biggest joke on its largest unofficial fan site and the internet at large. Find me any article or video related to GT where these flaws haven't been mentioned/discussed/mocked in some capacity.
Compare this to something like Forza Motorsport which never had any major missteps.
Second, sales numbers have dropped by a little more than half between GT5 and GT6.
GT5: 11,940,000
GT6: 4,880,000
I don't really think this drop is because it came when the PS4 released since I would think (hope) people would buy up a game if it was good regardless of which system it was on. In fact, PD now have two strikes since many consider GT6 to be worse than GT5, especially considering that it took two years for the game to receive its most anticipated feature, course maker. And even when it did arrive, it turned out to be a tablet app. A fact that PD never mentioned until it was released (this was the point where I've had enough).
So combine GT's damaged reputation and the seemingly related drop in sales, then add the competition on top, and I'm a little confused as to why it could still be a system seller. At the very least it is no longer as potent as it was on PS2.
Edit: The list I had here was a little much, and not really related to the argument.