[sarasi]

Greywolf, each writing system has its own order for listing its letters/characters. In English the vowels happen to be in the order a e i o u. The Japanese syllabary happens to start a i u e o. You say that the reason is that we write the vowels in alphabetical order- well, in Japanese they write the vowels in the established order for the Japanese syllabary.

As for why the vowels are in that particular order, well, we could ask the same thing about the Roman alphabet, why does e come after a in the list of vowels? Why not before a, or after u? Why does b come after a? At some point in history it was standardised in that way, just as in Japan the order of the vowels was standardised as a i u e o.