I'd like to know why you think that is?
-First (full) GT game with Online.
-First (full) GT game with night, weather effects.
-First GT game with visible damage.
-First (full) GT game with more than 6 cars on the track.
-Improved AI (emphasis on improved, not perfected)
-First GT game to feature Maserati, (officially) Lamborghini and Mclaren
-First GT game to implement visual customization outside of spoilers.
-Return (quietly) of Racing Modifications.
-Introduced track generator.
-etc. etc.
No, it wasn't flawless, perfect, amazing, but GT5 (along with GT5

) was a huge step forward for the franchise. A laundry list of things that we've been begging for for a decade were checked off in one and a half games.
GT5 is the perfect game, because PD will know where to go now. Where to take it further.
A year ago, I had a very long, drawn out, almost-nasty debate with another gentleman on here about whether GT6 would be on the PS3 or PS4. I was in favor of the PS3 and had a good deal of quotes and rumors that pointed to such... but even I have accepted that it is about 99% most likely going to be on the PS4, with the other 1% being that they announce that they're going to stop making games in general... or announce Tourist Trophy 2... or the world ends.