GT5 was made for the PS3 hardware specifically, as most recent PS3 first party games have been. The PS4 has very different hardware purely for the sake of making it get away from all of the problems developers had with the PS2 and PS3 early on. PD may have helped design it, but they wouldn't have done anything to compromise the system for other developers just to make it easier to port their stuff to. You're going to lose all of your optimization one way or the other when making the game for two systems; so it either suffers on the PS3 and has technical shortcomings that GT5 didn't have to make it simpler to port, or the PS4 version suffers because the game was largely designed for CELL and it's being made to run on an x86 platform. And either way it would be a massive expenditure of resources to get it on both systems at the same time; to say nothing about what Sony would have to do after the fact to keep support up when they split the game out over two systems.
That's why the only recent example of a similar thing being done was with a Gamecube game on the Wii (Twilight Princess), where both systems had nearly identical architecture so the porting process basically amounted to changing the control scheme.
