As a vegan, sometimes it's nice to eat at a restaurant and not have to go through the whole 20 questions with the server about whether food has animal products. So I may weigh other factors (like service or presentation or how "nice" a place is) as less important than when going to a restaurant with only one or two vegan options.
I've noticed Vegan and Vegetarian restaurants get high ratings, I think it's because the potential audience is so happy with the fact the place even exists (and gives them an alternative to cooking yet another meal or fighting with yet another menu) that the default is a high rating. Even very mediocre places around where I live have 4+ stars.
