I think the York sisters have been in the public eye (due to their parents - particularly their controversial mother, their fashion sense, their nearness in ages to William and Harry and perhaps just due to growing up in the era when the RF was seen differently to how it is seen today).
Where Zara shied from the public eye, they want it.
If James and Louise were a similar age to William and Harry (and frequently wore downright strange attire in public) we'd all be a lot more interested in them.