No official reason and you would think it easy to port... I get the feeling we may see them for sale in the future though
I think Devedander is on the right track here (no pun), unfortunately.
Also, the increased need to grind is related to this.
It seems that PD has had a general shift of philosophy since GT3~GT4.
I remember when they were all about creating a top-quality game, that was just UNMATCHED by any competition. Sure they have the best physics engine (I'd still pay $60 for JUST the nordschleiffe and an M3, and NOTHING but driving)..... but there's definitely something to be said about their shift in philosophy.
For example: there are two ways to enhance replay-value of a game:
1) make the career mode complex, evolving, challenging, and diverse. (Like GT4 for example..... TOOK ME FOREVER TO FINISH IT... and MINIMAL GRINDING)
2) make the career mode simple, repetitive, easier, and grind-happy. (I'M PRACTICALLY DONE WITH GT5... if you factor out my grinding, I would've been done even FASTER).
I think it's safe to say that PD has chosen #2. If comparing GT5's track list to the other GTs doesn't convince you, look at one of the MAJOR parts of update 1.05: you can now RESTART a race RIGHT AFTER you've just finished it!!!! WHY WOULD ANYONE WANT THAT?!!?!? I JUST FINISHED THIS RACE!!!!!!!!! It's a "GRIND" button essentially!
It' really sad that this is what they're resorting to. Instead of adding more content to fix an unfinished (but released) game, you make it easier for people to repeat events?!?!?
What's even more sad is that people are applauding this..... as if it's an improvement to the game. That's weak.