I guess I'm qualified to field that answer.
Truth is, I dunno. Maybe it's that I am fond of the centaur race on the mud personally, that my most notable (and pretty much only notable) character on the mud as a mortal was a centaur shaman. Perhaps it's that several imms in the past have been somewhat of patrons to different races, Sadr/Yenko being often a deep-elf and Meissa a halfling. Or maybe it's just my feeling that centaurs as a race are orderly and/or druidic in nature, both being factors I oversee. Besides, everyone else in the Pantheon is just a plain ol' boring Human on the list. How dull is that?
This isn't to say that Ain is ICly a centaur. I don't really think of him that way at all, and being that I can make him appear as any damn race on the who list, my personal affinity for centaurs makes it my first choice. Don't read too much into it, though. I just luff the horsies.