I'm guessing potentially cancelled pre-orders is a concern. Much less concern once multiple sku's are announced to transfer pre-orders to.
Another possibility - there could be ongoing talks with a company or two over cross-branding with a console (I forget the exact term) in commercials, marketing, etc. I doubt there's a realistic chance of going full blown exclusive, but I could definitely see MS or Sony, especially MS, lay out a few bucks to tie the names together in marketing. I could also see them lay out money for an exclusive feature, mission set, weapon / skin sets, etc. A few month delay would probably be perfect amount of time to finish off next gen plans and add a couple exclusives for anyone willing to pay I would guess.