She could of course do that. But I think also that she has to make herself famous first. We wouldn't be talking about her clothes that much if she kept on wearing unknown Danish designers, you know what I mean? I mean there are some princesses who don't really wear clothes from well-known designers, and its like we aren't really interested. We might say "o she's so elegant, she's the most elegant princess" but this doesn't translate to us buying clothes from the designer of that ballgown. Heck, we don't even ask who designed it! We just say it looks good and leave it at that. So at the end of the day those princesses might stick to designers from their own country, but because they haven't laid the ground work yet fashion-wise, they are ultimately ineffective as fashion ambassadors. (cf Q Rania - she wears quite a lot of designer clothes, and people like Valentino (and other designers whose clothes she has worn) hail her as an "icon"... I doubt that those designers would be praising her had she stuck with middle eastern designers... not many designers want to praise "lesser known rival's" creations).
And there is also the fact that at the end of the day, most of us are also suckers for brand names (e.g. this explains why when we post pictures of clothes we want Mary to wear, we stick to well-known names). Plus a big part of us being interested in the princess' dresses is the fairytale factor, by that I mean that we wish we could have what they have. The only real diffiuclty with us accessing those lovely designer clothes from well-known edsigners is the price, whereas with small-time Danish designers, we have to go to Denmark etc.. too "impossible", too many obstacles to take into account which is a no-no when you're dreaming of yourself wearing those clothes...so we don't spend that much time hankering for it.. .
So anyway, personally I think Mary is doing it now in order to put herself on the fashion world stage. Its like when you're starting a relationship, in the beginning you really want to impress and so you wear elegant clothes and clothes that will get you noticed. But later on, when she establishes a name for herself fashion-wise (by that I mean, people from America will start knowing about the CPss and consider her a good dresser) she should start wearing more and more Danish designs. I don't doubt that she will to be honest. Mary's not thick.