First if Att is so bad why do they have so many customers?
Second it's truth that Att covers 97% of population as well as Verizon. The map commercial was a lie. The Verizon map shows square mile. Go to Att site and see.
I believe both networks are close in regards to reliability however it's noted that Att has better performance.
Why do you think Sprint and t-mobile are not close to Verizon and Att? It's because their networks are inferior to Verizon and Att. If Att was that bad then they would be #3/4.
1). Because until now, AT&T was the only network you could get an iPhone on in the USA. That alone can explain why AT&T gained more customers over the last few years.
2). There's a lot more to "coverage" than what can be explained by looking at a coverage chart! There are other issues such as capacity, signal strength, reliability, etc. While AT&T's network is fine in many areas, there's no question that AT&T has many more trouble spots than Verizon.
3). Regarding performance, it's true that AT&T has better DATA performance, at least theoretically. But again, Verizon still outperforms AT&T in many areas because of capacity issues and signal trouble spots. Would you rather consistently get 1-2 mbps downloads, or wildly varying speeds that often runs much lower than that or where signal drips altogether? For many, this is a very real choice they have to make. In areas where AT&T is good, then yes indeed they would have the performance advantage.
4). Regarding T-Mo and Sprint, again they don't have the iPhone, nor they as readily available in as many areas so of course they don't have as many customers. Number of customers is not how one should rank the carriers. For example, US Cellular got best rankings in consumer reports, but they have a lot fewer customers because they are regional.
Bottom line is that one should choose a carrier based on how the network performs in the area they live, which can vary wildly.