Look at a piano. If every note had a sharp/flat, how would you know which note was which? The notes are based around the groups of three black keys and two black keys. This helps to easily identify notes on the piano. The guitar just took the notes from the piano.
This is, again, just my idea (The same question nagged me a while back, so I thought of this)
"You must stab him in the heart with the Bone Saber of Zumacalis... well, you could stab him in the head or the lungs, too... and the saber, it probably doesn't have to be bone, just anything sharp lying around the house... you could poke him with a pillow and kill him."
- Aqua Teen Hunger Force, The Universal Re-Monster
- Aqua Teen Hunger Force, The Universal Re-Monster