The "alpargatas" (espadrilles or espardeñas is its name in catalan), it is a traditional Spanish shoe with centuries of history, which is part of many of our regional costumes and which has been incorporated into fashion in Spain for decades. Men, women and children use it in summer because it is a light and comfortable shoe, which also prevents blisters that can be suffered with leather shoes in the high temperatures of the Spanish summer. Perhaps in other countries it is a "fashionable" shoe but in Spain it is something traditional.
Sofía has always liked fashion more, her style is that of girls her age in Spain and being at home with her mother and her fashion consultant has been able to choose and adapt everything she buys. It is normal that at her age she does not want to stand out for being the tallest, or that because of her height it is uncomfortable for her to walk in heels... her flat shoes are original designs and well chosen.
Leonor, perhaps because of her personality, perhaps because of her position as heiress, has always been less innovative with clothing. She has spent two years in Wales where the students dressed very informally, usually jeans, sweatshirts and trainers. If we add to this that her body has changed a lot in this time (and she still doesn't seem completely comfortable in public, with a tendency to cover her body) and that probably most of her clothes for events have been either chosen from photos or taken from her mother's closet... well, she hasn't quite found a defined style yet.