The only thing I can think is that lots of people were so sold on the iPhone 4 that they bought one even though they did not have good AT&T coverage where they normally live, work and travel.
People are always buying devices they think are great, but which do not really have the best coverage for them.
AT&T has very good coverage for the population in general, covering most of the concentrations of population.
But it's an indisputable fact that if you have to do a lot of travelling, your chances of always finding coverage are the best with Verizon. Verizon's not perfect, not by a long shot...but if you need geographic coverage more than you need high speed connectivity, then it's entirely possible that Verizon's just a better choice.
In current state I agree. However if AT&T is going to go through it's 3G expansion then they will be very close to Verizon, although Verizon even in AT&T's expansion will still have more 3G coverage.
Why do you think AT&T wasn't able to keep up toe to toe with Verizon 3G coverage?