Probably a few reasons:
-AOK runs on average computers that, by now, pretty much everyone has access to at home.
-AOK is based on a more associable and well-known topic (Middle Ages) while AOM is about pantheons and epics that not as many people know or are appealed to.
-Since its release in 1999 (a whopping four years ago, huge time in the gaming business) AOK has developed an unprecedented worldwide following. These fans are probably still convincing their friends to go out and buy AOK.
-As for buying AOK, it is much cheaper. Perhaps another factor in its outselling of AOM.
Ensemble truly ought to be commended on producing a game that, four years after is release in the previous century, is still near the top of the sales lists. As many have said before, AOK combined with its expansion pack, TC, produces one of those "timeless" games that, like Starcraft, simply never seem to lose thier appeal.