If Verison joined the mix, there will be price wars that would potentially eat into how much Apple gets paid by each carrier.
Why? As long as the iPhone is the "must have" device, Apple is in the driver's seat. It would simply dictate the price the carrier must pay to carry the phone. No pay = no sale. Simple.
Just like that snail commercial forever soured Intel on working with Apple?
For better or for worse, you don't F with Steve Jobs. Verizon is ***** with Steve Jobs. Steve Jobs doesn't like to be F'ed with.