Because they have to write a version suitable for the new console. PD has suggested they'll be doing so, but I doubt you'll see anything until later in the year.
To help with this. Gran Turismo has ALWAYS released 2 games per system. PSX had GT1 and GT2, PS2 had GT3 and GT4. PS3 has GT5 and GT6 some may argue GT5P as a game as well. I don't because how limited it was. But there ya go. As far as I also know and read.
Kaz has been developing GT6 for PS3 since GT5 was released to the public. Hence why when the PS4 was announced. GT6 was pretty much already done for the PS3 game engine. Ya know.
They needed a spike in old, profitable PS3 units to carry the low ball PS4 intro pricing. They asked a number of developers to release new titles on the PS3 this holiday season to spike sales.
They are also using games like this as an up sell point to move people to the PS4.
"See how much better it looks" will be a mantra heard for the next year or two for all of these titles.
This is all planned out. The only issue is that Drive Club was delayed, otherwise the GT6 to GT6.5/GT7 program is right on schedule. If anything, I anticipate the port to the PS4 sooner than the 2 years I originally figured they wanted to run with. Figure 1 year committed DLC and a 1 year marketing phase for the up sell for Christmas 2015.
However, with all the NotWork issues and the game glitches and problems on release I don't see them waiting. Instead of trying to waste time fixing the PS3 version properly, I see them "consoling" PS3 GT6 owners by trying to get them to jump ship to the PS4. Once they get the game there, the community will be their biggest sales force... They'll fix the big stuff and keep their promises on the PS3, but like GT5 many problems won't ever see a real fix, they'll be "features" of GT6.5/GT7... On the PS4.
Patience is a virtue, but for PD and SONY it's also a marketing tool. They have this thing where they believe people can wait for it. With little for competition this has been true, but competition is coming. Time is no longer the ally or marketing tool it used to be for them. It's my opinion, but this is likely the first time PD actually doesn't have the time to waste making a move, they honestly cannot afford a prologue/GT5/GT6 time frame for GT6.5/GT7.
Guess time will tell...