We know he got it through an American army contact shortly after the war. Why he never destroyed it remains a question. The former Queen would never have had it publicised her son stated that he feels history needs to be faced warts and all. He is not happy about his grandfather's membership of the NSDAP before his marriage to the Dutch crown princess but it is part of history. He could easily have followed his great grandmother's example. It has been said that Queen Wilhelmina smoked the fireplaces when she went through the archives of her father, halfbrothers and husband. The current king as a trained historian is more respectful of the evidence. He wants the real truth to come out and has opened the private family archives up until the 6th of september 1948, the day his grandmother Juliana succeeded to the throne.
That means still quite a lot remains hidden. Probably because three of the queens daughters are still alive. Besides the actions of a monarch also have way more weight than those of a royal consort.
"The Dutch government has confirmed the authenticity of a Nazi party card held by Prince Bernhard, prince consort for decades after World War Two."
Wonder why he never destroyed it?
https://www.bbc.co.uk/news/world-europe-67017311
648