Cingular/AT&T (I wish they would finally make the switch) made a deal with Apple to be the exclusive provider for the iPhone. They did this without seeing a prototype, so it was a leap of faith. Considering Apple's recent track record, who could blame them. As you can see in this article, Did Verizon Pass on the iPhone, it could have been a revenue sharing issue. Up until now, wireless carriers have been able to control the manufacturers when it comes to device features, etc. The deal with Cingular is unprecedented because it allowed Apple to build the iPhone to their specifications. Take Motorola for instance, Microsoft and Verizon dictate how long their splash screens appear on the Moto Q.
I do think Verizon will eventually carry the iPhone or perhaps iPhone 2. They are too big of a network to ignore as Apple will want to expand their reach.