The reason why we get the scythe of elune and Allerias bow, is because they are seeded as either lost or mythic weapens. Allieria is gone, so her bow is something which is neautral. Her connection to the alliance is not something which the game has showed in action. The scythe of Elune is a legendary artifact in its own right already, and it has never been used. Since it has never been used to battle the Horde or the Alliance, it is a neutral thing.
I just don't really see the blades of Varian Wrynn as something legendary or mythic. Sure, they have a unique design, but they have not shown to hide any special power or ability. They are on the same power lvl as Saurfangs cleaver. Proberly a good weapen, but not really legendary.