The difference is the starting point. All examples you gave are born royals, there was no any doubt about that and some of these pictures were 'artistic' (the Duke of Kent looking in the mirror) or remained regal (the old Princess Louise in all her finery). Princess Marina and Princess Margaret made some doubtful choices, but then it a royal lady breaking out of the frame.
Charlène still can be filed under the category "has-no-idea-what-she-is-doing-in-Monaco", is still not seen as THE Princess of Monaco when we compare her with sister-in-law Princess Caroline. So the starting point, the framing, is completely different. Princess Charlène could have set one monumental photoshoot and for once and for all frame her as Son Altesse Sérénissime la Princesse de Monaco, Marquise de Baux. She could have done so by wearing historic jewels which were worn by generations of monegasque royal ladies, by wearing her country's highest Order, by appearing stately and regal in the pictures, preferably again referring to the 700 years of her House's rule (posing near an old painting, near the throne, at the marble steps of the cour d'honneur of the palais princier, whatever).
Nothing of all this. All pictures show a lady in all sorts of modelling poses, with expressions implicating seduction and not even a historic jewel but the Océan Diadem which has, despite the big name (Van Cleef & Arpels) a quite nouveau riche look, I have to say. Charlène stood before a wide open goal and she missed the easy shot, totally.