Because the PS2 was designed somewhat logically. The GS only renders and performs certain effects that are mostly only implemented before the rendering process beginns (i.e. AA). Also, the bandwidth isn\'t all that much (1.2 GB/s), so devs should only send displaylists and textures to achieve good results.
On Xbox, you have a GPU that has to do way more than just rendering. The NV2a even has to perform complex perspecticve transformations - a task which on PS2 is handled by one vector-unit. All in all, bandwidth requirement on Xbox should be much higher because they all connect via a UMA. That\'s also a reason to why AA is not used in 90% of Xbox games (yet).