I would think marketing/advertising is the big reason. ATT puts way more marketing effort into the phone because it's exclusive to them (especially if it's 5 years as has been noted some places, not sure if confirmed or rumor) vs if two or more companies are able to get it branded it's just another device in the pile. Sure it's got Apple's name on it and all, but they aren't going to invest in stuff the way they would with the exclusive. ATT is setting up some elaborate store displays among other things. Plus there's that backend stuff again (for visual voicemail, etc.), makes that more palatable for ATT to perhaps cover the whole cost of implementing.

Secondary reason: simplicity selling at the Apple Store. If it's on two or more carriers they have training and setup infrastructure for two or more sets of plans, plus two inventory SKUs for the same device (one for each brand). Not a chance. Yeah, other stores sell many phones for multiple providers, but do they do a good job of it? Or, alternatively, they could play favorites at the Apple Store and choose one of the two. How would that go over at the non-favored provider? Doesn't work well. To translate the Apple Store model to selling the iPhone, it has to be one provider. That will be complicated enough with all the different voice plans, although perhaps there is something unique in that arena as well? We don't know for certain yet what the plans are....

(I was posting this last night right at the FT maintenance window. Oops! Had to wait until now.)