Boing_737, I think that happens because owners of unusual aeroplanes have no motivation to offer them to mag reviewers unless they wish to sell the aeroplane. If the reviewer is fair in their assessment, that shouldn't matter.
The same situation applies with the more usual source of flight test subjects; a dealer or agent for a current type offering it to a mag for flight test. The object here is to get the type onto the radar of the readers, some of whom may be potential purchasers of that type.
Same thing really; publicity.
SSD