Wait, wat? Kebab is middle-eastern food with hundreds of years of history. 'Doner kebab' (the popular one in a bread) is Turkish. German sausage has nothing to do with it.

There are lots of ways you can serve kebab/shoarma and none of them are 'wrong' per se. The popularity of doner and roll is probably just a matter of convenience for customers and business owners. The sauce thing was just an observation, and frankly I don't know how what local influence made it served that way in Europe.