It's because of the power and class your ZR1 is. Your ZR1 is around 1000 HP, I assume, and being that it's class is a sports/super car, there's only a few matches for it. The Cerbera Speed 12 is around a 1000 HP super car, as is the Veyron.
I liked the difficulty level setting in GT5p :/
sometimes you want to do easy races and sometimes you want to do hard races, but in GT5 you're stuck to racing medium races...