But why are they treated differently then eg. Maxima or Mary. They too are commoners, but they do not seem to receive this treatment. Is it that they were not born in the country they now live in?
In the book "Aan het Hof" by Remco Meijer and Jan Hoedeman is written that Queen Beatrix preferrably wanted her eldest son to have a partner "with a certain standing"
and from a foreign country. The first serious relationship of the Prince of Orange (Emily Bremers) did not meet these requirements. Máxima Zorreguieta at least was "foreign".
According Meijer and Hoedeman, the Queen found it "not desirable" that someone who comes from the Netherlands itself, from a normal Dutch family, with Dutch neighbours, Dutch classmates, member of Dutch sports clubs, Dutch colleagues, in short: "with a Dutch past all over" becomes the future Queen of the Netherlands. In her opinion the distance which is, in her eyes, a "necessity" for a proper workings of a monarchy would disappear with such an alliance.
When Máxima Zorreguieta engaged with the Prince, immediately all sorts of media and correspondents went to Buenos Aires, New York and Brussels and tried to trace everything they could about her. The geographical distance, the language barrier and the fact that the lady in question was brought up "protected" (on an expensive private school) and soon left for New York made that there was little information to pick up and the focus soon became on the father with his past in the military dictatorship.
Mathilde d'Udekem d'Acoz was a Belgian but a lady who is raised on the Château de Losange in Villes-la-Bonne-Eau and hardly counts as a "normal" lady the Belgians can relate to. Stéphanie de Lannoy is a non-Luxembourgian and has the same background as Mathilde, being raised at the Château d'Anvaing in Hainaut. Both are clearly not the postman's daughter, so to say, and come from the "natural habitat" of the royal family.
Mary shares the same "luck" as Máxima to have her background and roots at the other side of the world. Also Alexandra and Marie are foreign, by the way.
Mette-Marit and Letizia are the exceptions here. Catherine too, but she is still in the goodwill-mode as young married couple with a lovely kid. The vitriolic British press can slash her, like they did with Sarah or even Diana during certain periods.