Err... cause it's awesome and Miyagawa's a genius?
as for MySQL, well that's just what I use, shouldn't be too hard to make it use a different DB tech as it uses the standard DBI interface, and as far as I know it supports several types.
Or would you prefer working with files? The first aXML only worked with files as I didn't know about SQL when I wrote it. The current version doesn't have any file accessors yet really beyond the inclusion routines.
I could whip some up and send you an updated engine package if you want?