Well, they were tired of running the business. Maybe they tried to sell it, but no takers. Second or third generation didn't want to run it. Lease was up and landlord demanded more than they were willing to pay.
I regret to add that if the DQ brass had been with me on my last visit to a filthy, fly-ridden, worn out restaurant, they would have shut them down on the spot. Gotta believe that “tired of running” neglect had something to do with it.