Because titles created by Brisith and Spanish monarchs for their relatives are different. In Britain, members of the RF are granted hereditary noble titles, like any other subject (in the first generations royal titles, but then not). In the Spanish case, the King does not grant a Dukedom. The member of the RF is simply "authorised to use" the title. In such a a case, the authorisation can be reversed. So, Infanta Cristina ceased to be a Duchess when his brother stripped her of the title. But , in the past, it was different with the Dukedom of Sevilla. It was created by King Fernando VII for his nephew, Enrique, later creaated an Infante. When he later conspired against the Crown, he was stripped of his title of Infante ( the Queen's prerrogative), but not of his Dukedom. It is an hereditary title still existing.
Another example: the King can strip Infanta Margarita of her title of Duchess of Soria, but not of her title of Duchess of Hernani.
In Spain, we had noble titles and titles of the Royal House. Such a difference does not exist in Britain. Maybe a good idea for the future.
Well at least the Spanish monarch has a direct control and decision in who gets what title within the wider royal family and ALSO if he/she can take away that title once it's given - an ability *not* allowed to British monarchs. In fact, even worse situations in certain *other* existing European monarchies where the the parliament has completely taken over the initiative and prerogative of deciding who gets a title at all.
I do not understand. What would the fresh approach be?
Could be no ducal title, just leave daughters and any future sons-in-law as they are. Or wait until sons-in-law have proven themselves worthy over time, and then honor them in some fashion. It was awkward what happened with Cristina’s ducal title and in retrospect it would have been better if it had not been granted. And JC’s other son-in-law soon proved to be unworthy as well.