If you're writing the history of the world, and you know it's going to take you quite a while, you start writing what you know best first. Your own local history.
PD isn't the only one guilt of this kind of thing. English language World History books will harp on about the accomplishments of Greeks, Romans, Angles and then American colonists, and only pay lip service to Chinese, Indian and American Indian culture.... well, they used to... over time, they pick up little pieces here and there and become more complete.
That's the way GT is. They started out Japano-centric, doing what they knew best, and then started including stuff from other markets, making the game's appeal wider and wider (big step backwards in GT3 for not including muscle cars and classics). GT4 is a big step up from GT3 in terms of representation of other countries and maufacturers, both in terms of the number of manufacturers and the number of models represented. So why are we complaining? Merely because there are less of them than Japanese cars.
The GT encyclopedia/history will never be complete. There are just too many cars, and there's too little time for them to test/model every single one of them. They will continue being Nippon-centric, not by choice, but because there are simply so many Japanese cars available to them and so easily at hand, while they have to spend more time and money hunting down and testing "foreign" cars. A lot of the testing requires manufacturer support.
Which is why Toyota, Ford and BMW have a lot of cars in the game, because they lent time, resources and data to PD. Pissed-off because the Corvette Z06 in the game hasn't been updated for years? GM may be partly to blame. Dodge Chrysler and Ford seem to get their new cars in just fine.
I'm not putting down guys who want more American/European/Australian cars... I want them, too. But we can't lay all of the blame for their absence on PD.