A ponytail is too casual for Kate. Why should she scrape her lush hair back in that way?
I'm not saying anyone should do anything, I just think it would be nice to have a bit of variety. One of the reasons I last grew mine was to be able to style it differently from time to time. She looks so pretty with it up, I'm just surprised she doesn't do it more.
And conversely, I think loose hair is pretty informal as well but we'll just have to agree to differ 