The point is that what Audi did was throw a lot of money and put up a whole department in the factory for developing the rally car, to use it as a marketing tool. Since then teams like Lancia, Toyota, Mitsubishi, Subaru, Peugeot, Citroen and VW have also been successful in their own eras, but they haven't upped the level of effort as much as Audi did. VW was just a lot better than the other teams during their era, and they had the best driver of the era (same as Citroen the decade before). The rules alone controlled better how much you could do development during a homologation period and what are the limits of a car whereas in the group B days it was quite the Wild West.