I'm sure it was something they signed w/ Sony.
Yeah, Rockstar signed a time-limited exclusivity agreement with Sony. You can only buy GTA games on the PS2 for a certain amount of time, and then after that, Rockstar is free to port them to other consoles. The agreement used to be exclusivity to PS2, with the except of the PC, period. However, the agreement was modified so Rockstar could release the Double Pack on Xbox. And now there's a GTA game on the GBA, so it's pretty clear Sony's agreement is limited only to consoles--the PC ports take extra time to code in features like custom skins, the mp3 player, resolution changes, EAX, etc.
(I'm assuming it'd look better on the ex-box...)
You assume correctly. Just look at the Double Pack. It even has mp3 support, thanks to the hard drive. Sony doesn't want GTA on the Xbox, since it's bad enough they've been losing console sales since August due to Halo 2's impending release--their short-supplying the channel because of the PStwo didn't help either.