Sony lets them just do their own thing.
I grew up playing GT on ps1 and ps2, but they take their sweet time making games. I got the 360 in late 2006 and fell in love with Forza 2 when it came out. Then 3 and 4 built on that. Now we're at Forza 7 like clockwork and have an amazing Forza Horizon series between main releases.
GT just became irrelevant to me. The time PD takes to make them doesn't produce a huge quality gap between their rivals. Im sure it's still popular though.
I bought an import of GT1 after reading the Edge review. Modded my PS1 to play it. Even had to buy a new TV with RGB Scart input. That's how much I was into GT from the off.
A few years later I was even the editor of a racing game site for IGN that was primarily focussed on GT. I spent hundreds of hours on those first three games. I was very into Gran Turismo.
I don't think I've spent more than five hours on GT6. For me the franchise has lost its way. While GT4 was okay I remember reviewing it saying there are signs that the game is going the wrong way and its inability to sync with Sony hardware launches was an issue too.
Polyphony does amazing car models. Super replays. Great lighting. The best photo mode results. But the game? The game's not there. And given how long we spend looking at tracks rather than car models, the tracks are pretty poor.
This decline coincided with the rise of Turn 10. Dan's team doesn't get everything right. But they seem to mostly have their priorities right (points systems and qualifying aside - I'm not after a hardcore game but these should be in). One of Turn10's major triumphs that isn't talked about as much is how they've got a very convincing physics model that's still playable and fun through a controller. So many developers struggle with this - Project Cars (of which I'm a backer) has never managed this.
I wish we had a GT that was as strong as Forza. But Polyphony just seems in a world of its own - perhaps like a lot of Japanese game development these days. I'll be buying GT:S (I'm in the beta) but FM7 is the game I know I'll be putting many more hours into.