There are few teams in the Top14 who have played in towns other than where they are. Toulon sometimes play in Marseille and I think Racing played in Brussels once (maybe I'm wrong?) and now in Lille. Why, I don't know. Maybe you're right, Primrose, that the authorities want to promote rugby in the far north.
Sometimes Toulouse play at the football stadium and Stade Français and Racing at the Stade de France, but of course, that's not playing out of town, so to speak.