there may also be times when the original callsign of an airline can't be used, because it could indicate that the defunct airline was back alive and well. Case in point, the 3 incarnations of National Airlines. First one used 'National', then went belly up. The name for the airline was bought by another entity, in which 'SunKing' came out (like the logo on the tail of their DC10s). Then they went belly up. Then the Rio/Harrah's, and Michael Conway bought the name, and used 'Redrock' for its callsign (the airline was based in Vegas; a scenic tourist trap called Redrock Canyon is about 15 miles NE of the airport).
So it could also depend on the history of the airline as well.