Because everyone knows the white iphone is for girls and/or people who want everyone to know they spent the extra money regardless of the fact that they only need 2.3 GB of storage....
Commence the onslaught of hundreds of replies debating the merits of this ridiculous argument by people who wish they could afford the white iphone but convinced themselves that the 8 GB was "totally enough space dude. I'll just change my songs every week or something"
In all seriousness, I think it's because they didn't want to give the impression that the white one was available for both models, but my answer will hopefully bring hours of entertainment to this discussion....