One does not fulfill his or her obligation of lighting the menorah if it is 20 Amos above since it is not easily seen.
The Rambam in Hilchos Shabbas 17:15 writes that a beam over a mavoi with pictures and designs works even above 20 Amos since it is recognizable and attracts attention.
Why don't we apply the same logic by the menorah. If one is 20 Amos above why can't he hang a big sign to attract people to it?
On page 227 of the link which was posted by Avrohom Yitzchok he gives a difference which seemed quite compelling to me. When it comes the beam, there is no idea to see the beam for its own sake, it is rather to be able to recognize that the boundary is there, so you see the sign, and that is good enough.
Whereas with the Chanuka candle and a Sukka, you have to see the actual candle or the actual Schach, not something incidental to them.
