My suspicions on an answer for your question is that this isn’t a plane for everyone.
This era aircraft only appeals to a specific crowd, so those that purchased it only to leave it in their hangar, probably account for some of the ratings.
I will say that if you like flying aircraft of that vintage, the PMDG DC-6 will keep a smile on your face.
There really is nothing else in this class of airliner that has been made quite as thoroughly.
It is a joy to fly, but it does take some learning to master it.