One thing you learn about the restaurant industry during expansion is maintaining talent and culture is not easy. Alot of these restaurant cultures succeed if there is either a top level culture passed down by two or more founders, or its instilled from a family culture
This happened with a local burger chain - once they grew beyond 3 locations that were relatively close together, the quality at all of them dropped significantly. My guess is that the founder put managers in place who weren't as passionate and went more hands-off, leading to the the slide in quality.
The odd one manages it but too many just end up with low talent, low invested staff slopping out bad food that has none of the implementation or taste that funded the expansion and the whole things slowly collapses in on itself.