It would change their mainline design (the new "country-specific" look most probably not looking as good as the original one) + having a different design (only

It would change their mainline design (the new "country-specific" look most probably not looking as good as the original one) + having a different design (only for the US market, especially involving the front part which is extremely important in the general look) might probably have to be marketed under a different name (e.g not "A4" but "A4us") with all kind of repercussions on in/direct costs of all kinds (storage space for replacement parts, marketing, research on future evolution, ...). Additionally, accepting a request from a specific country might trigger a chain-reaction from other countries, multiplying what stated above by the nbr of countries (and I personally don't think that it would be a linear curve as at the same time the costs for coordination between the multiple lines would rise).