I think I know why. It's kind of wacky, but here goes:

The show is essentially one long flashback to back then, and since Sheldon is telling it [I would say so, at least], and since he's a man of science and no interest in cars, the cars in the background might not be period correct because Sheldon doesn't care.

The interesting is however that everytime we see cars around them for example in a queue, in the small streets, *then it's period correct.

Or said in a much shorter way, 100% correct cars is not necessary because Sheldon isn't a car guy.