Most likely because the original sold poorly on the Gamecube, and Lucasarts is not willing to take the risk of another failure. It costs money to port a game, money that is usually returned through sales. However, they don't want to loose money should the game not sell well (based on sales of Jedi Outcast, of course), so they simply don't bother. The ones that really want the game can always pick it up on another console or PC.
Trust me, it's not that amazing a game. It's good, but not enough of a step up from Jedi Outcast to warrant a full purchase. You're probably safer when it's down in the bargain bin.