Fewer than 500 words...
AT&T sucks, Verizon is better! Uh, maybe not.
Targeting AT&T for complaints about coverage, or voice reception, or incomplete 3G coverage is accurate, all those charges are true, but the next thing in the argument needs to be compared to what? Verizon's voice is better, but it's not better because Verizon has put more money into their network or because Verizon is modernizing faster. Verizon's voice is better because they use CDMA instead of GSM. CDMA is higher power, uses fewer towers and has significantly longer reach than GSM, but GSM has wildly more capacity, does better at a bunch of lower level things like encryption, and handles data far better. Those commercials where a Verizon user needs two phones to do data and voice simultaneously aren't a joke, that's a real failing and it's not what iPhone users expect.
So, for the initial launch of the iPhone and its maturing AT&T needed the iPhone and Apple needed AT&T, thus AT&T was the only reasonable choice. Verizon wouldn't play ball, AT&T would and continues to, and the other carriers in the US are too small or financially unhealthy to be considered. But now we're past that, right?
Why is the iPhone on AT&T when Verizon is better?
Many people complain that AT&T is the reason they either won’t get an iPhone or that iPhone service is bad. So, why is the iPhone exclusively on AT&T?
When Apple was creating the iPhone the company realized that it would need a partner that would be able to deal with a completely new product. The sales model would be different, the phone would cost more, and the users of the phone would tax the nascent data side of cell communications. Apple couldn't just drop the iPhone on any and all carriers, because at the time of release and as things have continued the iPhone pushes the boundaries of what's currently available and what will be available. Apple could not risk that the iPhone would be with a carrier that wouldn't or couldn't keep up with the evolving uses Apple hoped for the product.
AT&T was willing to essentially refocus their company to match Apple's vision of the future of cell phones and persistent and pervasive data access. AT&T believed that working with Apple would completely change their company and the market they served. They were right, clearly.