I think that it's because Luthien's part Maiar that she is greatest of the Eldar. Even with Galadriel learning from Melian it's not the same as being her daughter. I think it's really interesting to note that the Royal Dunedain have (no matter how small) some Maiar blood in them.
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Though here at journeys end I lie in dakness buried deep, beyond all towers strong and high beyond all mountains steep, above all shadows rides the Sun and Stars forever dwell:I will not say the Day is done nor bid the Stars farewell!
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