There was an accident with one Oldtimer (i´m not shure if it was a DC-3 or DC-2) in The Netherlands, after which the Authorities restricted the Use of Oldtimer-Airplanes for Passenger-Carrying Flights.
DDA was not able to get the DC-4 Certified for Passengers and it was to Expensive to Operate the Plane without the Income from the Passengerflights.