Because the Greek Alphabet it decended from was roughly similar (Alpha, Beta, Gamma,Delta) which in turn was because the ancient Semitic alphabet it came from was roughly similar ( Aleph, Bet, Gimmel, Dalid ) In the Semetic alphabet each letter represented (and kind of looked like) a corresponding word (Aleph= oxen, Bet=house) I do not know why they were put in the order they were back then.
not discouraging more/new input on this, but meanwhile, get started on this section of the FAQ (link on sidebar)