It's a very good question, but I'm afraid I don't know the answer. I can speculate, of course, but I've never asked this question of anyone who would actually know (e.g. manager/member of staff at a restaurant) because I don't want to annoy them by asking too many questions, and I usually have plenty of questions about the food!

I think there may well be a perception among some restaurateurs that there isn't enough demand, but I don't know how close this perception comes to reality.