For the brief period when some games came out with both Xbox and Xbox 360 editions, sales were already seriously stacked in favor of the 360 version. Given that there is no longer any penalty to users of modified Original Xbox consoles due to the lack of Xbox Live support, any game released for that console would also have serious concerns with pirating.
Studios looking to create less expensive titles can still work with the 360's superior specs thanks to Xbox Live Indy Games and Xbox Live Arcade, so budget concerns can't even justify going back a generation.
Long story short: The Playstation 2 still has enough of a user-base for some developers to justify (a few) new PS2 games. The Original Xbox does not. Users that could afford to buy new games upgraded to the 360 almost half a decade ago.