We all know that AT&T has the exclusive rights to distribute Apple iPhone. Many people complain on AT&T network performance, they expect better if iPhone on Verizon network, but AT&T have the exclusive rights.
There are news reported that Verizon was the first mobile operator that Apple talk to for iPhone distribution, but Verizon think that Apple want to much control (as usual) and too much profit share cut. That's how AT&T has came in later.
You can read the Techcrunch story AT&T is the company you sleep with, Verizon is the company you marry.