If your question is "why is it so small" we really cannot give you an accurate answer; however,
AT&T has a relatively new 3G network...it was launched around the time of the original iPhone 3G and was only released in major areas. Launched services in large areas is NOT uncommon. When Verizon released their 3G coverage, it was limited to larger states and larger cities. Verizon Wireless extensively built their network and it has taken about 2 years for them to have 3G coverage nationally. AT&T has had their 3G network about a year.
They are expanding, but I'm frightened that with the upcoming launch of LTE on most major carriers (other than Sprint) AT&T may remove their focus from 3G altogether. This is also not uncommon. Since Verizon Wireless has announced they will be supporting LTE for Data and then later voice, they have also announced they have seized all CDMA coverage expansion.
I have seen AT&T's 3G coverage get larger, but it is still small and over time may or may not improve.
If you feel you're iPhone is not what you wanted, maybe you should consider other carriers (like Verizon or Sprint) in the future.
Notably, T-Mobile has the smallest 3G network in the United States.
Also notable, Verizon Wireless launched the first 3G network within the United States and they were the second provider to launch 3G worldwide (meaning they launched their 3G network in the United States after Vodafone launched theirs in the UK).