Because you need separate cooking utensils, prep areas, storage, and serving-ware for the various sorts of -isms, as well as allergies. Cross contamination is tough enough in a normal commercial kitchen, almost impossible when you throw in a few restrictions. For example, peanut and vegan, much less gluten free and vegetarian.
The slice of the market for serving more than one of those is not sustainable. If you want to specialize it is doable, but still not economically feasible.