If I understood correctly, the crown prince would choose to marry Marie over these two noble girls as a way to bridge the unity of her cocuntry into his. And even without her boosted up stats (serously though, she has Mary Sue stats but the character is so good that it doesnt matter. Kind of like the stats look good on her instead of stats making her look good) she is a better candidate for queen than these two.
Although mentioned briefly, it was hinted that Marie was the beloved princess of her country because of her not so secret charity work.
I knid of want for the prince to figure out her true identity soon but I know that if he does then the story could end not long after. And I believe none of us want that.