Right now, every iPhone in the world uses GSM technology, regardless of the carrier. Adding Verizon [who use a different technology, CDMA] would mean building, testing, and forecasting new hardware for a CDMA model. It would also have a significant impact on the well-oiled Apple Retail machine.
Every time a new iPhone launches, there are lines out the door. Add a CDMA phone to the mix, and you either have two lines, or complicate the process by waiting for each customer to decide between carriers. If the customer changes his mind mid-purchase, the Apple Retail rep has to go get the other model—adding time to the transaction and running tempers higher for everyone in line. In-store returns will inflate with gifted iPhones that were bought for the wrong carrier. The retail implications of a second wireless chipset are in many ways negative.
A good rule of thumb for understanding Apple's strategy in the 2010s: if something won't work in an Apple Retail Store, there's no way in hell they'd do it.