Same reason I would get a DS over a PSP.
The Xbox is nothing over the PS2 for me, only thing I like the Xbox is their hard drive but that is about it (Xbox Live is not even supported in my country, PS2 online on the other hand is ... damn pity that I have yet to see a PS2 HD on sale) and I play more PS2 games that Xbox games.
I dont see any decent Xbox exclusives yet, I see a lot of cross platform games that I can get to run better in my PC (that is nothing much) if I cared.
Besides I stand over my impressions over the next consoles, the PS3 and Xbox "2" will have a small technology gap since they will be released around the same time and as Sony is moving towards its DVD/MP3 player/God knows what/Console combo the Xbox decided to make a console instead of a stripped down computer with Nintendo appearing to wanting to inovate how games are "played".
Honestly I am tired of this "tech race" that simply downplays games to tech demos ... Fable is not better that FF X-2 because of the Xbox "capabilities" (both games stink, even if in diferent ways) and I seen Silent Hill fans saying that SH4 looks better in the PS2 that in the Xbox.