Why is it that she's not a high fashion model? She definatly is not in the same league as others who are not even as pretty.
Maybe it's her look - slightly more commercial/sexy
Or it could be the handling of her career - she did a LOT of bridal ads/catalogue, which isn't good for any "image" - she also did a lot of tackier ads, like fornarina and sisley.
I think someone who did those sorts of ads aren't going to be in american vogue any time soon..