It was the desire of the lawmaker (the Dutch Government) that the titles
Prince (Princess) of the Netherlands and/or
Prince (Princess) of Orange-Nassau remain closely connected with the person of the King.
Back then in the 1960's the
Hoge Raad van Adel (High Council of Nobility) advised against the use of the title
Prince (Princess) of Orange-Nassau for the children of the younger daughters of Queen Juliana. They advised to follow the rule that by lack of males the most senior female heir is treated as if she was a male and the titles pass via her like it would have passed in the male lineage. However Queen Juliana insisted and stated she did not want to create A-Princes and B-Princes: all her grandchildren were equal to her. Despite her attempts of course the three sons of Princess (Queen) Beatrix had (and have) a total different status than their cousins. The wish of the old Queen was the father of the thought but did not work in practice.
The solution of 2001 (Constantijn) and 2004 (Friso) to create a new branch of the dynasty belonging to the Netherlands Nobility with the surname
Van Oranje-Nassau van Amsberg, the title
Count (Countess) van Oranje-Nassau and the predicate
Jonkheer (Jonkvrouw) van Amsberg was a more durable one. Their descendants in the male lineage will remain members of the titled nobility ánd the wish to connect the princely titles close to the person of the King is honoured. Imagine that Princess Irene and Christina were married with consent, then we would now have 22 Princes (Princesses) of Orange-Nassau walking around...
New ennoblement in the Netherlands is possible for members of the Royal House. Ennoblement in Belgium is possible. Look at Mathilde and her family, all were "promoted" from the baronial rank (by right of firstborn) to the comital rank (by right of all). No problem.