GT5 didn't have the DBS, Cinque, GT500, and a ton of other cars that similar games have. Plus, some people ( :whistle: ) prefer the boldness of the '07 GT500. Remember, not everything has to be the latest and greatest - that's certainly the case with American cars, as the last memorable American cars (excluding the CTS-V, ZR1, ACR, and the like) were created before the 1970's.
EDIT: ^TDU2's been a work in progress for over three years, and they don't have the time or resources to A) get insider info as to what the future models will be (like GT5) or B) license and create new models.
American omissions aren't really a big deal for me, as most stuff made here is crap (everyone give a "lol" for Saturn...). The CTS-V is a loss, however, having used it in Simbin's Race On. The ZR1, too, but the Caddy and 'Vette are the BASICALLY only world-class cars not on the list.
-Leadfoot