That is a very good question indeed. That the diadem was unwearable, allez... that seems understandable. But the collier would, stll today, have been an eyecatching and extremely impressive piece. Very suitable for the young Princesses in the 1960's or for Queen Máxima now.
If it is really broken up too, it is beyond belief. "Bernhard, let us break both the massive diadem ánd the collier, for our daughters." "What you want, Juliana..." Looking at the diadem below, and thinking about the earhangers, little broches and maybe a bracelet here and there, to me it seems all bijoux together match the quantity of the diadem, not of the diadem ánd the collier.