There were supposed to be safety inspections, training, and other regulatory procedures which were not carried out properly or not at all.
The ferry was illegally modified and still passed the safety inspection when it was commissioned after being renovated.
The ferry was overloaded and the freight was not properly locked down.
The people manning the Coast Guard stations were negligent in their duties. The actions of the Coast Guard and Korean Navy did not positively or effectively assist in the rescue.
The situation room at the Presidential compound which is supposed to be used in these kinds of disasters was not used.
And so on...
The BBC film focuses on these points. Thank you BBC for doing something mainstream Korean media cannot seem to do.