There's a few good reasons why the X-box didn't get one in America and why the Gamecube hasn't gotten one in Europe.
Basically, the X-box and the PS2 sell better in Europe and in America, the X-box performs the worst. (Out of the 3 that is, the system itself performed terribly in Japan)
It doesn't really help Gamecube sales when people wont actually release games on it though. Same for X-box.
Porting the game onto another system is simple really - as is converting it. Because there is an X-box version coming out here, chances are sooner or later, America will get an X-box version too.
The Gamecube might get AH in Europe before long too. Usually, companies that are releasing for all 3 consoles that arn't aiming for a simultaneous release and don't expect massive guaranteed sales (Most stuff not released by EA in otherwords) tend to go onto the X-box and PS2 first and once they've made their profit - get chucked to the Cube owners.
I can wait for my Cube version, but really it's quite depressing in a way that Zoo Digital arn't keen on releasing it for all three right away.
I dunno why the Game hasn't received the full release on all 3 systems in any country yet. It's definitely worth it and worse games have been distributed for all 3. A case of money... maybe - but the game is brilliantly designed and is one of the few REAL games produced any more -fighting among all the liscenced EA crap.
Hopefully, everyone will get the game for all their systems soon enough.