Since nobody else seems to have jumped on this one....
The official answer from Lewis Carroll (the author), is "Because it can produce a few notes, tho they are very flat; and it is never put with the wrong end in front!"
The answer is actually an afterthought; Lewis Carroll didn't have an answer in mind when he wrote it. Other classic answers include "Because they both come with inky quills," and "Because Poe wrote on both."
Feel free to supply additional answers.
Hmmm...
