Well, yes and no. Its materials were tougher and modern cars are more prone to crumpling, but also that modern cars (not even counting measures like seatbelts and airbags) do a much better job of protecting the cabin thanks advances in frame design and unibody construction vs. old body on frame models. Newer cars disperse kinetic energy of the crash into crumple zones, whereas older cars would sustain heavier damage because they weren't designed to absorb as much.
Even though this video is 10 years old, it shows the difference in crash technology. On the 2009 car, more of the cabin is preserved, whereas the 1959 cabin is almost completely flattened, despite both of them shredding through their hoods/engines. Christine could conceivably regen from that, but since the car would be in worse shape than a modern car in a similar crash, she'd have to regen from more damage or regen would take more time than if she were a more modern car.
It should also be noted that this technology started to become more widespread in the early 80s. Even if KITT were a regular car, his frame likely would've been constructed with these measures already in place. But then consider her frame is layered with armor plates and its hull is a molecular-bonded shell, enough to act as a moderate bomb shield to protect humans and still come out unscathed.
Likewise, I *think* Christine had limits to her regen, in that it seemed to grow slower and slower over time with the more damage she took, though I'm not sure if that was linked to her owner.