Because Poe wrote on both. Pax-C

Because the notes for which they are noted are not noted for being musical notes. (Puzzle maven Sam Loyd, 1914)
Because Poe wrote on both. (Loyd again)

This reference claims that it is because they can produce a few notes that are flat. (originally there was no answer and that was made up by Lewis Carroll after the book came out).

http://www.stra…

tho many have come up with different answers, "Because they both come with inky quills." ect the joke it that there isnt an answer, its funny watching people attempt to find the answer, its in the same theam as sending some one to buy a left handed screwdriver or striped paint.