No I didn't mean to suggest that all ruins were unsuccessful but rather that often cities were abandoned when they were no longer viable for some reason .
Sometimes they exhausted their resources. sometimes they out grew their resources and moved. Sometimes they were forced out by enemies. Sometimes they left due to religious beliefs. Sometimes a natural disaster or sickness ! Side note today's huge populations wouldn't exist if farming technologies hadn't vastly improved the amount of food production per acre. Technologies allow survival in areas otherwise nonviable. Less advanced cultures either adapt to what is available or have to leave for greener pastures or pass out of existence. Sometimes it is a combination.

As an off side I love the appropriate add below . "looking for underground bunker? Find it nearby with local.com "