Both Stark sisters require eloping for marriage, same with Myrcella Baratheon because none of them have their father in-game so you can't ask for their hand, you must build relationship until any of them agrees to marry by eloping. You can do the same for any marriage prospect but since with most you can just ask for their hand, which costs less time to achieve, and also since eloping pisses their families off and makes you lose some relations with their respective faction, eloping is usually a last resort or for these ladies you can't marry any other way.