Ive often wondered about this. My theory is that the Jewish market isnt big enough for a small niche establishment. Subways is limited to subs (obviously) they also have a “healthy” flair thus no cole slaw no french fries etc, their simply isnt a big enough market for this. There are very few truly niche kosher restaurants consider: Chinese stores arent truly “chinese” they usually carry sushi for example.
Just a theory.