Nope, it's not because of the middle class or anything related. Simply because the food is terrible and has gotten worse and worse in recent years. People are realising there are better alternatives out there, not just ones that reply on brand recognition with little else going for them.

Add the fact it's easier to find out how good or bad a restaurant is via reviews, and you've got a situation where terrible chain restaurants are going the way of the dodo as people chose alternatives with decent food.