The flight crew were not licenced to pilot this aircraft. Simple.
Really simple? Are there still training organisations around which have instructors current on this type? Are there training aircraft available? (approved) Flight simulators? Are there any examiners current on the type? Is the manufacturer maintaining the training material up to date? Or even the pilot handbooks? Does the licence mean anything?
It might be a pure beurocratic paperwork issue, you can probably have pilots which can perfectly handle such aircraft but are lacking the paperwork, you probably can also have pilots perfectly licenced but havin no clue how to operate them.
For 1950s vintage aircraft it does not make much sense to aply 2018s regulation for flight crew licencing.
So it is a simple fact with a difficult truth behind...